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This document contains Chapters 14 to 15 CHAPTER 14 ACOELOMORPHA, PLATYZOA, AND MESOZOA CHAPTER OUTLINE 14.1. General Features A. Cephalization 1. Sessile animals survive well with radial symmetry. 2. Concentrating the sense organs on the head is an advantage to active animals that seek food; this provides an anterior and posterior end and bilateral symmetry. B. Position and Biological Contributions 1. These are the simplest animals with primary bilateral symmetry. 2. The majority are triploblastic with all three germ layers. 3. They have a solid body without a coelom; they have acoelomate bodies. 4. Typical acoelomates have only one internal space, the digestive cavity. (Figure 14.1) 5. The region between the epidermis and digestive cavity is filled with parenchyma; parenchyma is a form of packing tissue containing more cells than extracellular matrix. 6. Specialization of organs provides the organ-system level of organization. 7. Some phyla are typical acoelomates, having a gut cavity lined with endodermally derived cells. 8. Other phyla possess pseudocoelomate bodies which contain an internal cavity surrounding the gut, but the cavity is not lined with mesoderm, as in coelomate animals. (Figure 14.1) 9. The gut lining of a pseudocoelom is endodermal cells. The pseudocoel may be filled with fluid or a gelatinous matrix with some mesenchymal cells. 10. The pseudocoelom shares some functions of a coelom but the most likely function is as internal circulation in the absence of a true circulatory system. 11. Animals sharing a particular body plan do not necessarily form a monophyletic group. 14.2 Phylum Acoelomorpha (Figure 14.2) A. Characteristics 1. Acoelomorphs are small flat worms less than 5 mm in length. 2. Typically live in marine sediments; few are pelagic. 3. Some species live in brackish water. 4. Most are symbiotic but some are parasitic. 5. Group contains ~350 species. 6. Members were formerly in Class Turbellaria within phylum Platyhelminthes. 7. Two orders, Acoela and Nemertodermatida, now represent two subgroups in phylum Acoelomorpha. 8. Acoelomorphs have a cellular ciliated epidermis. 9 Parenchyma layer contains small amount of ECM and circular, longitudinal, and diagonal muscles. B. Digestion and Nutrition (Figure 14.3) 1. Some have a digestive system from a mouth to a tube-like pharynx followed by a sack-like gut. 2. No anus. 3. In many acoels, the gut and pharynx are absent, so the mouth leads into either an endodermally derived mass of cells or syncytial mass. 4. Phagocytotic cells digest food intracellularly when food is passed into temporary spaces. C. Reproduction 1. Acoelomorphs are monecious. 2. The female produces yolk-filled eggs called endolecithal eggs. 3. Following fertilization, some or all cleavage events produce a duet-spiral pattern of new cells. D. Nervous System 1. Other defining features proposed for acoelomorphs are biochemical (patterns of neurotransmitters) or rely on cellular ultrastructure such as formation of a network of interconnecting rootlets from epidermal cilia. 2. Acoelomorphs lack a “true” brain. 3. They have a radial arrangement of nerves instead of a ladder-like pattern seen within Phylum Platyhelminthes. E. Phylogeny of Acoelomorpha 1. Phylogenetic studies describe acoelomorphs as early-diverging bilaterally, symmetrical triploblasts. 2. They have only four or five Hox genes. 14.3 Clades within Protostomia 1. Most triploblastic metazoans are divided among two large clades: Protostomia and Deuterostomia 2. Protostomia is divided into two large clades: Lophotrochozoa and Ecdysozoa. 3. Before the use of molecular phylogeny studies, scientists grouped the protostomes on the basis of body plan. 4. Molecular phylogenies group acoelomate and coelomate taxa together within the protostomes. 5. Members of Ecdysozoa possess a cuticle that is molted as their bodies grow. 6. Members of Lophotrochozoa share either an odd horse-shoe shaped feeding structure, the lophophore, or a particular larval form called the trochophore. a. Trochophore larvae are minute, translucent, and roughly top-shaped. b. They have a prominent circlet of cilia and sometimes one or two accessory circlets. c. They occur in the early development of marine members of Annelida and Mollusca, and are assumed to be the ancestors of such groups. A. Clade Platyzoa (Figure 14.4) 1. The Platyzoa is a group of lophotrochozoan protostome phyla. 2. There is not consensus among phylogenetic analyses for the phyla forming the Platyzoa. Some phylogenies show that Platyzoa contain Platyhelminthes, Gastrotricha, and four phyla in Gnathifera. 14.4. Phylum Platyhelminthes A. Characteristics (Figure 14.5) 1. Are commonly called flatworms. 2. Flatworms vary from a millimeter to many meters in length. 3. Some flatworms are free-living; others are parasitic. 4. Some authors argue that the phylum Platyhelminthes is not a valid monophyletic phylum. 5. The parasitic clades share an external body covering called a syncytial tegument or neodermis. 6. Platyhelminthes is divided into four classes (Figure 14.6): Turbellaria, Trematoda, Monogenea, and Cestoda. a. Class Turbellaria contains free-living flatworms along with some symbiotic and parasitic forms; most are bottom dwellers in marine or freshwater. 1). Freshwater planarians can be found in streams, pools, and even hot springs. 2) Terrestrial flatworms are limited to moist places. 3) Class Turbellaria is depicted as a paraphyletic taxon and awaits a complete revision. b. All members of Monogenea and Trematoda (flukes) and Cestoda (tapeworms) are parasitic. B. Form and Function 1. Epidermis and Muscles (Figures 14.7, 14.8, 14.9) a. Most turbellarians have a cellular, ciliated epidermis on a basement membrane. b. Rod-shaped rhabdites swell and form a protective mucous sheath. (Figure 14.7) c. Most turbellarians have dual-gland adhesive organs. (Figure 14.8) 1) Viscid gland cells fasten microvilli of anchor cells to the substrate. 2) Secretions of releasing gland cells provide a quick chemical detachment. d. Some turbellarians, and all other members of this phylum, have a syncytial epidermis; nuclei are not separated by cell membranes. e. The non-turbellarians lack cilia and have a tegument; they form the subphylum Neodermata. f. Under the basement membrane, muscle fibers run circularly, longitudinally and diagonally. g. Parenchyma cells fill spaces in the body; they are noncontractile portions of muscle cells. 2. Nutrition and Digestion (Figure 14.10) a. Cestodes have no digestive system; others have a mouth, pharynx and an intestine. b. In planarians, the pharynx can extend through the mouth that is mid-ventrally located. c. The intestine has three branches, one anterior and two posterior. d. This gastrovascular cavity is lined with columnar epithelium. e. The mouth of trematodes and monogeneans opens near the anterior end; the pharynx is not extensible; the intestine ends blindly and varies in the degree of branching. (Figures 14.11, 14.19) f. Planaria Feeding 1) They are carnivorous and detect food by chemoreceptors. 2) They entangle food in mucous secretions from glands and rhabdites. 3) They wrap themselves around prey and extend the proboscis to suck up bits of food. g. Monogeneans and trematodes feed on host cells, cellular debris and body fluids. h. Proteolytic enzymes from the intestine are secreted for extracellular digestion. i. Phagocytic cells in the gastrodermis complete digestion at intracellular level. j. Undigested food is egested back out the pharynx. k. Cestodes rely on the host’s digestive tract and they absorb digested molecules. 3. Excretion and Osmoregulation (Figures 14.10, 14.11) a. Flatworms have protonephridia that could be used for secretion or osmoregulation. b. Beating flagella drive fluids down collecting ducts, forming a negative pressure drawing fluids through a network or weir. c. The wall of the duct beyond the flame cell bears folds or microvilli to resorb certain ions and molecules. d. The majority of metabolic wastes are removed by diffusion across the cell wall. e. Collecting ducts join and rejoin until they empty at nephridiopores. f. Marine turbellarians lack these units because they do not need to expel excess water. g. Monogeneans have two excretory pores opening laterally near the anterior. h. Flame cell protonephridia are present also in the parasitic taxa. i. Ducts of trematodes open into an excretory bladder that opens to a terminal pore. j. Cestodes have two main excretory canals to each side that run through the worm. (Fig 14.23) k. Metabolic wastes are removed largely by diffusion through the body wall. 4. Nervous System (Figure 14.10B) a. The subepidermal nerve plexus resembles the nerve net of cnidarians. b. One to five pairs of longitudinal nerve cords lie under the muscle layer. c. More derived flatworms have fewer nerve cords. d. Freshwater planarians have one ventral pair of nerve cords forming a ladder-type pattern; the brain is a bilobed ganglion anterior to the ventral nerve cords. 5. Sense Organs (Figure 14.10A) a. Active locomotion favored cephalization and evolution of sense organs. b. Ocelli are light-sensitive eyespots in turbellarians, monogeneans and larval trematodes. c. Tactile and chemoreceptive cells are abundant, especially in the ear-shaped auricles. d. Some have statocysts for equilibrium and rheoreceptors for sensing direction of water currents. e. Sensory nerve endings are in oral suckers, holdfast organs and genital pores of parasitic groups. 6. Reproduction and Regeneration (Figures 14.10 − 14.12) a. Fission 1) Many turbellarians constrict behind the pharynx and separate into two animals. 2) Each half regenerates the missing parts; this provides for rapid population growth. 3) Some do not separate immediately, creating chains of zooids. b. Regeneration 1) If the head and tail are cut off, each end grows the missing part; it retains polarity. 2) An extract of heads added to a culture of headless worms prevents regeneration. c. Some asexual reproduction occurs in intermediate hosts; (see life cycles to follow). d. Nearly all are monoecious but cross-fertilize. e. Endolecithal eggs with spiral determinate cleavage are typical and ancestral. f. Some turbellarians and all other groups have female gametes with little yolk; yolk is contributed by separate organs, vitellaria. 1) Vitelline ducts bring yolk cells to the zygote; this is ectolecithal development. 2) A cleavage pattern cannot be distinguished; the zygote and yolk cells surrounded by eggshell move into the uterus. g. Male Structures 1) One or more testes are connected to vasa efferentia that connect to one vas deferens. 2) The vas deferens runs to a seminal vesicle. 3) A papilla-like penis or extensible cirrus is the copulatory organ. h. Turbellarians develop male and female organs opening at a common pore during the breeding season. i. After copulation, eggs and yolk cells are enclosed in a small cocoon and attached by a stalk to plants. j. Embryos emerge and resemble little adults; however, some marine forms are ciliated and free-swimming larvae. k. Larval trematodes emerge as ciliated larvae to penetrate a snail, or may be eaten by a host. l. Cestodes hatch only after being consumed by a host; many different animals can serve as intermediate hosts. m. Trematoda, Monogenea, and Cestoda are united into a single clade called the Neodermata. C. Class Turbellaria (Figures 14.13, 14.14) 1. Turbellarians are mostly free-living and range from 5 mm to 50 cm long. 2. Except for polyclads, endolecithal turbellarians have a simple gut or no gut and a simple pharynx. 3. Polyclads have a folded pharynx and a gut with many branches. 4. Larger polyclads have more highly branched intestines. 5. Members of order Tricladidia are ectolecithal and have a three-branched intestine. 6. Very small planaria swim by cilia. 7. Others move by cilia and gliding over a slime track secreted by marginal adhesive glands 8. Rhythmical muscular waves pass backward from the head. D. Class Trematoda 1. All trematodes are parasitic flukes. 2. Most adults are endoparasites of vertebrates. 3. They resemble ectolecithal turbellaria but the tegument lacks cilia in adults. 4. Adaptations for parasitism include: a. penetration glands, b. glands to produce cyst material, c. hooks and suckers for adhesion, and d. increased reproductive capacity. 5. Some trematodes retain ancestral characteristics of alimentary canal and reproductive, excretory and nervous systems. 6. Sense organs are poorly developed. 7. Subclass Digenea a. Nearly all have an indirect life cycle with the first intermediate host being a mollusc. b. The definitive or final host where sexual reproduction occurs is a vertebrate. c. A second or third intermediate host may be required in the life cycle. d. They parasitize a wide range of hosts and most parts of most systems in hosts. e. General Digenean Life Cycle (Figure 14.15) 1) The egg passes from the definitive host in excreta and must reach water. 2) The egg then hatches into a free-swimming ciliated larva, the miracidium. 3) The miracidium penetrates the tissues of a snail and transforms into a sporocyst. 4) The sporocyst reproduces asexually into more sporocysts or many rediae. 5) Rediae reproduce asexually into more rediae or into cercariae with tails. 6) Cercariae emerge from the snail and penetrate a second intermediate host or encyst on objects to become metacercariae, or juvenile flukes. 7) The adult grows from a metacercaria when it is eaten by the definitive host. f. Serious parasites of humans and domestic animals are digeneans. (Table 14.1) g. Sheep Liver Fluke 1) Fasciola hepatica was the first digenean whose life cycle was described. 2) The adult fluke lives in the bile passage of the liver of sheep and other ruminants. 3) Eggs are passed in feces and miracidia hatch and penetrate snails to become sporocysts. 4) After two generations of rediae, cercaria encyst on vegetation and await being eaten by sheep. 5) When eaten, the metacercariae grow into young flukes. h. Clonorchis sinensis: Liver Fluke in Humans (Figure 14.11) 1) This is the most important human liver fluke; it is common in China, Japan and Southeast Asia. 2) It also infects cats, dogs and pigs. 3) The adult fluke is 10–20 mm long with an oral and ventral sucker. 4) The digestive system includes pharynx, esophagus and two long intestinal ceca. 5) Excretory system has two protonephridial tubules with branches with flame cells. 6) The nervous system has two cerebral ganglia and longitudinal cords with transverse connectives. 7) Males have testes, two vasa efferentia uniting to a vas deferens, a seminal vesicle and ejaculatory duct, but lack a cirrus. 8) Females have a branched ovary, and a short oviduct joined by ducts from seminal receptacle and vitellaria at the ootype. 9) An ootype is surrounded by Mehlis’ gland; the uterus then extends to the genital pore. i. Clonorchis Life Cycle (Figure 14.15) 1) Adults live in bile passageways of humans and other fish-eating mammals. 2) Eggs containing a complete miracidium are shed into water with feces. 3) The eggs hatch only when ingested by snails of specific genera. 4) The miracidium enters snail tissue and transforms into a sporocyst. 5) A sporocyst produces one generation of rediae, which begin differentiation. 6) Rediae pass into the snail liver and continue embryonation into tadpole-like cercariae. 7) Cercariae escape into water and swim until they contact a fish in the family Cyprinidae. 8) Cercariae bore into fish muscles or under scales, shed their tail and encyst as metacercariae. 9) A mammal eating raw fish dissolves the cyst and young flukes migrate up the bile duct. 10) A heavy infection can destroy the liver and result in death. 11) Control is by destroying snails and thoroughly cooking fish. j. Schistosoma: Blood Flukes (Figures 14.16, 14.17) 1) Over 200 million people have schistosomiasis, infection with blood flukes. 2) It is common in Africa, South America, West Indies, and the Middle and Far East. 3) It is unusual insofar as sexes are separate. 4) The male is larger with a gynecophoric canal to embrace a smaller female. 5) Three species account for most human schistosomiasis: S. mansoni in venules of large intestine, S. japonicum in venules of small intestine, and S. haemotobium in venules of urinary bladder. k. Schistosoma Life Cycle 1) Eggs are discharged in human feces or urine. 2) In water, eggs hatch as ciliated miracidia; they must contact a particular species of snail to survive. 3) In the snail, they transform to sporocysts that produce more sporocysts. 4) Daughter sporocysts produce cercaria directly; cercariae escape from the snail and swim until they contact bare human skin. 5) Cercariae pierce the skin and shed their tails; they reach a blood vessel and make their way to the hepatic portal blood vessels. 6) After developing in the liver, they migrate to their appropriate site. 7) Eggs released by females are extruded through the gut or bladder lining and exit with feces or urine. 8) Eggs that remain internal become centers of inflammation. 9) Eggs of S. mansoni and S. japonicum can damage the intestinal wall; S. haematobium damages the bladder wall. 10) Control is achieved when people dispose of wastes hygienically, but is difficult in poverty. 11) Schistosome dermatitis (swimmer’s itch) occurs when cercariae penetrate an unsuitable host such as a human. l. Paragonimus: Lung Flukes (Figure 14.18) 1) Paragonimus westermani is a lung fluke that parasitizes humans, pigs, rodents, etc. 2) Its eggs are coughed up in sputum, then swallowed and eliminated in feces. 3) Zygotes develop in the water and miricidia penetrate a snail host. 4) Within the snail, miricidia give rise to sporocysts, which develop into rediae. 5) Cercariae are shed into the water and ingested by freshwater crabs. 6) Metacercariae develop in freshwater crabs; human infection occurs by eating uncooked crabmeat. m. Some Other Trematodes 1) Fasciolopsis buski lives in human intestines in India and China; it is contracted from eating raw aquatic vegetation. 2) Leucochloridium produces remarkably colorful sporocysts in snails’ heads, thus attracting birds to eat them and continue the life cycle. E. Class Monogenea (Figure 14.19) 1. Monogenetic flukes were originally placed in Trematoda; some authorities now argue they are sister taxa, both having a posterior attachment with hooks. 2. Monogeneans are external parasites of fish, especially gills, but a few are found in bladders of frogs and turtles. 3. Monogeneans have a direct life cycle in a single host. 4. The oncomiracidium attaches to a host by posterior hooks. 5. The posterior hooks may become the posterior attachment organ of the adult, the opisthaptor. 6. Opisthaptors vary widely (hooks, suckers, clamps) to withstand the force of water flow. 7. Some are serious economic problems in fish farming. F. Class Cestoda (Figures 14.20, 14.21; Table 14.2) 1. Tapeworms have long flat bodies composed of scolex for attachment to a host. 2. The scolex is followed by a linear series of reproductive units or proglottids. 3. The scolex is a holdfast head portion with suckers and hooks. 4. Tapeworms lack a digestive system. 5. Muscles, excretory and nervous systems are similar to other flatworms. 6. They lack sensory organs except for modified cilia. 7. As with Monogenea and Trematoda, the tegument is syncytial and has no cilia 8. The entire surface of cestodes is covered with projections similar to microvilli seen in the vertebrate small intestine; these microtriches increase the surface area for food absorption. 9. Subclass Eucestoda a. Aside from two orders of lesser importance, all have proglottids and are polyzoic. b. Larvae have six hooks on the scolex. c. The chain of proglottids is called a strobila. d. Proglottids originate in the germinative zone just behind the scolex. e. A proglottid is usually fertilized by another proglottid in the same or different strobila. f. Shelled embryos form in the uterus; they are either expelled or the whole proglottid is shed. 10. Proglottid formation is not “true” segmentation; replication of sex organs is not equivalent to metamerism in annelids, etc. 11. Nearly all cestodes require two hosts; the adult is parasitic in the digestive tract of the vertebrate 12. Over 1000 species of tapeworms are known, infecting almost all vertebrates. 13. Most tapeworms do little harm to the host. 14. Taenia saginata: Beef Tapeworm (Figures 14.22, 14.23) a. This tapeworm lives as an adult in the alimentary canal of humans; the juvenile form is found in intermuscular tissue of cattle. b. Mature adults can reach over 10 meters in length with over 2000 proglottids. c. The scolex has four suckers but no hooks. d. Gravid proglottids (with shelled, infective larvae) pass in feces. e. Excretory canals run from scolex along proglottids; flame cells attach to excretory ducts. f. Nerve cords from a nerve ring in the scolex run along proglottids. g. Each mature proglottid has muscles and parenchyma plus male and female organs. h. This order contains vitellaria in a single vitelline gland. i. Gravid proglottids usually crawl out of feces. j. Proglottids rupture as they dry; embryos are viable for five months and are picked up by grazing. k. Life Cycle 1) Cattle swallow shelled larvae that hatch as oncospheres. 2) Oncospheres use hooks to burrow through the intestinal wall into blood or lymph vessels. 3) When they reach voluntary muscle, they encyst to become bladder worms (cysticerci). 4) When the infected meat is eaten, the cyst wall dissolves and the scolex evaginates to attach to intestinal mucosa. 5) New proglottids develop in 2–3 weeks. 6) Infected persons expel numerous proglottids daily. l. Infection can be avoided by eating only thoroughly cooked beef. 15. Taenia solium: Pork Tapeworm (Figures 14.20, 14.24) a. This tapeworm lives as an adult in the small intestine of humans; juveniles live in muscles of pigs. b. The scolex has both suckers and hooks on the rostellum. c. If eggs or proglottids are ingested, the embryos migrate to organs and form cysticerci. d. Cysticercosis commonly occurs in eyes or the brain causing expected symptoms or death. e. Infection can be avoided by eating only thoroughly cooked pork. 16. Diphyllobothrium latum: Fish Tapeworm a. Adults are found in intestines of humans, dogs, cats and other mammals; immature stages are in crustaceans and fish. b. It is the largest cestode of humans, reaching up to 20 meters long. c. Fish tapeworms may occur when people eat raw fish. 17. Echinococcus granulosus: Unilocular Hydatid (Figure 14.25) a. Adults parasitize dogs and other canines; juveniles infest many mammals. b. Humans may serve as an intermediate host. c. A juvenile is a special cysticercus, a hydatid cyst, that grows for up to 20 years to a huge size. d. The main cyst maintains a single chamber; inside daughter cysts bud off with thousands of scolices, each able to produce a worm if eaten. e. Surgical removal is the only treatment. 14.5 Phylum Gastroticha (Figures 14.26, 14.27) A. Form and Function 1. Approximately 450 species and can be found in fresh, brackish, or salt water. 2. The body is usually elongated with a convex dorsal surface bearing a pattern of bristles, spines, or scales, and a flatted ciliated ventral surface. 3. There are no specialized respiratory or circulatory structures. 4. The digestive system is complete and comprises a mouth, a muscular pharynx, a stomach-intestine, and an anus. 5. The nervous system includes a brain near the pharynx and a pair of lateral nerve trunks. 6. Gastrotrichs are typically hermaphroditic although the male species of some is so rudimentary that they are functionally parthenogenetic figures. 14.6 Clade Gnathifera (Figure 14.27) 1. Gnathiferans possess small cuticular jaws with a homologous microstructure. 2. Numbers of pairs of jaws vary within the clade. 3. Gnathostomulida, Micrognathozoa, and Rotifera are tiny, free-living, aquatic animals. 4. Acanthocephalans are wormlike endoparasistes living as adults in fishes or other vertebrates. 5. Rotifera and Acanthocephala are presumed sister taxa, together forming a clade called Syndermata. a. Both groups have eutelic syncytial epidermis. b. This grouping is still controversial. 14.7 Phylum Gnathostomulida (Figure 14.28) A. Characteristics 1. They are delicate worm-like animals, less than 2 mm long 2. Over 80 species of jaw worms in 18 genera have been described. 3. They live in crevices of sediment and silt and endure low oxygen; they are often very common. 5. The epidermis is ciliated but with only one cilium per cell. 6. They glide, swim, and bend the head side to side. 7. They feed by scraping bacteria and fungi from the substratum with a pair of jaws on the pharynx. 8. The body is acoelomate with a poorly developed parenchyma layer. 9. Sexual stages include males, females and hermaphrodites. 10. Fertilization is internal. 14.8 Phylum Micrognathozoa (Figure 14.29) A. Characteristics 1. Micrognathozoans are tiny animals that live interstitially (between sand grains). 2. The body consists of a two-part head, a thorax, and abdomen with a short tail. 3. The cellular epidermis has dorsal plates but none ventrally. 4. These animals move using cilia and have a unique ventral ciliary adhesive pad that produces glue. 5. It has three pairs of complex jaws. 6. Simple gut. 7. Anus opens to outside only periodically. 8. Reproductive system is not well understood. a. Only female reproductive organs have been found. b. May reproduce parthenogenetically. c. Cleavage and subsequent development have not been studied. 14.9 Phylum Rotifera (Figures 14.30, 14.31) A. Characteristics 1. Rotifers have a ciliated crown, the corona that beats like a rotating wheel. 2. There are about 2000 species of rotifers. 4. Aquatic species are mostly benthic but some are pelagic. 5. Rotifers are highly diverse in color, size and shape; some are colonial. 6. Floaters are globular, creepers are elongated and sessile forms are vase-like. 7. Many endure desiccation and temperature changes by encystment. B. Form and Function—External Features 1. A rotifer body has a head, trunk and tail; only the corona is ciliated. 2. The ciliated corona or crown surrounds a nonciliated central area with sensory bristles and a mouth. 3. The corona is often a pair of trochal discs; beating of the cilia help in feeding and locomotion. 4. Some have a secreted cuticle and all have a fibrous epidermis layer, and a lorica that may be case-like. 5. The narrow foot has one to four toes and it may be retractile; it attaches with pedal glands that secrete an adhesive. C. Internal Features 1. Under the cuticle, a syncytial epidermis secretes cuticle and bands of subepidermal muscles. 2. The pseudocoel is large, filled with fluid and a network of mesenchymal ameboid cells. 3. Digestion a. Coronal cilia sort out larger unsuitable particles. b. The mastax is a muscular pharynx equipped with hard jaws, the trophi. c. Trappers have a funnel-shaped area around the mouth; side lobes fold inward to entrap prey. d. Hunters project trophi to seize prey. e. Salivary and gastric glands secrete enzymes for extracellular digestion. f. The stomach absorbs nutrients. 4. A pair of protonephridial tubules has flame cells and empty into a common bladder. 5. The bladder pulsates and empties into the cloaca. 6. Osmoregulation is important in both freshwater and marine species; water enters by the mouth. 7. A bilobed brain is dorsal to the mastax with paired nerves leading off to the organs. 8. Sensory organs include eyespots, sensory bristles and papillae, and ciliated pits and dorsal antennae. 9. Reproduction (Figure 14.32) a. Rotifers are dioecious; males are smaller than females. b. In some classes, males are unknown, and in others, males occur only briefly. c. Female systems may provide yolk to developing ova by cytoplasmic bridges. d. Ovaries and yolk glands may be combined as germovitallaria. e. Bdelloidea females are parthenogenetic, producing diploid eggs that hatch into diploid females. f. Monogononta females produce diploid amictic eggs that form diploid females, or haploid mictic eggs that, if not fertilized, become haploid males. g. Males have a single testis and ciliated sperm duct running to a genital pore and copulatory organ. h. Mating is by hypodermic penetration; sperm are injected into the pseudocoel of the female. i. Females hatch with adult features and mature in a few days; males are mature at hatching. 10. Phylogeny of Rotifera a. Recent molecular work questions the taxonomic affinity of some groups and distributions may be an artefact of morphological characteristics rather than taxonomic similarities. b. According to traditional classification, Rotifera has three classes, but some authorities would rather put Seisonidea and Bdelloidea to orders within a class called Digonata. c. Others divide the phylum into two classes: one containing the seisonids and the other containing the bdelloids and monogononts under the name Eurotatoria. d. At present, Acanthocephala is the sister taxon to Rotifera.\ 11. Classification Class Seisonidea Class Bdelloidea Class Monogononta 14.10 Phylum Acanthocephala (Figure 14.33) A. Diversity 1. All spiny-headed worms are parasites in the intestines of vertebrates. 2. Over 1100 species are known; they occur worldwide and parasitize fish, birds and mammals. 3. Its proboscis has rows of recurved spines that penetrate and may rupture host intestines. 4. Larvae develop in crustaceans or insects. B. Form and Function 1. The body is somewhat flattened. 2. The body wall is syncytial and its surface has minute crypts to increase surface area. 3. About 80% of the tegument is a lacunar system of fluid-filled canals that may distribute nutrients and remove wastes from muscles. 4. There is no heart; the lacunar fluid apparently serves this function. 5. Both longitudinal and circular body wall muscles are present. 6. The proboscis with hooks can be inverted into a proboscis receptacle by retractor muscles. 7. Two lemnisci may serve as fluid reservoirs when the proboscis is retracted. 8. There is no respiratory system. 9. If present, the protonephridia with flame cells perform excretory functions. 10. They lack a digestive tract and absorb all nutrients across the tegument, which bears some enzymes. 11. The worms constantly phosphorylate glucose so a gradient remains to constantly absorb more. 12. Males have a pair of testes; during copulation, sperm are ejected into the vagina and travel into the pseudocoel. 13. Female ovarian tissue breaks up into ovarian balls that rupture and float free in the pseudocoel. 14. A funnel-shaped uterine bell receives developing shelled embryos and passes them to the uterus. 15. Shelled embryos discharged in feces do not hatch until eaten by an intermediate host, often grubs. 16. No species normally parasitizes humans. 17. Acanthocephalans penetrate the intestinal wall with spiny proboscis; remarkably little inflammation on host wall, but pain of infection is intense. 18. Larval acanthors burrow through beetle intestine and develop into juvenile cystacanths in the insect hemocoel. C. Phylogeny of Acanthocephala 1. Divided traditionally into three classes: Archiacanthocephala, Eoacanthocepala, and Palaeacanthocephala. 2. Recent molecular work has suggested that the phylum status of the Acanthocephala is unwarranted, and that they may be a class of highly derived rotifers. 14.11 Phylum Mesozoa (Figures 14.34, 14.35) A. This group is considered a “missing link” between protozoa and metazoa. B They have a simple level of organization: they are minute, ciliated, and wormlike animals. C. All mesozoans live as parasites in marine invertebrates. D. Most are composed of only 20 to 30 cells arranged in two layers. a. These layers are not homologous to the germ layers of other metazoans. E. Two classes, Rhombozoa and Orthonectida, are so different that some authorities place them in separate phyla. 1. Rhombozoans a. Live in the kidneys of benthic cephalopods. b. Adults are called vermiforms and are long and slender. c. Inner, reproductive cells give rise to vermiform larvae. d. When over populated, reproductive cells develop into gonadlike structures producing male and female gametes. e. Larvae are shed with host urine into the seawater. 2. Orthonectids a. Parasitize a variety of invertebrates. b. Reproduce sexually and asexually; asexual reproduction consists of a multinucleated mass called a plasmodium. 14.12 Phylogeny A. Most phylogenies place members of phylum Acoelomorpha as the sister taxon to all other Bilateria, having a different embryonic cleavage patterns, in the way their mesoderm forms, and in the structure of the nervous system. B. Studies of ribosomal genes suggest that ancestral protostomes split from ancestral deuterostomes and then protostomes later split into two large groups: the Ecdysozoa and Lophotrochozoa. C. Evolutionary relationships within Lophotrochozoa are still in flux. Lecture Enrichment 1. Provide either specimens or slides to compare size, structure, habitats and relative complexity of the different invertebrates. 2. Compare the body structure of the flatworm and the roundworm, with emphasis on body layers, digestive tract, and reproduction. 3. More of these parasitic diseases are tropical. Slides or other audiovisuals can illustrate the ravages of schistosomiasis, fluke infestations, tapeworms, etc. Many cultural habits have evolved to circumvent ingestion of these parasites. Solicit the benefits of food preparation habits, hygiene, plumbing, etc. Some require a minimal level of affluence not attained in poor regions of the world. Commentary/Lesson Plan Background: Again, few students will be directly familiar with members of these groups aside from specific targeted biology labwork in high school and introductory college biology. Students from tropical countries should be able to relate the serious symptoms and social effects of the parasitic forms. Few North American students are aware of the extent that our social and food habits, plumbing systems, clothing and footwear, etc. help us avoid many parasitic infections. Misconceptions: Again, many students will believe evolution can only add complexity and will not recognize the ongoing evolution of these organisms, many of which did not establish their current parasitic relationships until the evolution of the birds or mammals that are now their specialized or sole host. Schedule: Providing the details of the many parasitic cycles and their devastating effects in tropical countries would add a third day. HOUR 1 14.1. General Features A. Cephalization B. Position and Biological Contributions 14.2 Phylum Acoelomorpha A. Characteristics 14.3. Phylum Platyhelminthes A. Characteristics B. Form and Function C. Class Turbellaria D. Class Trematoda E. Class Monogenea HOUR 2 F. Class Cestoda G. Classification of Phylum Platyhelminthes 14.4 Phylum Mesozoa A. Characteristics B. Phylogeny 14.5. Phylum Nemertea A. Characteristics ADVANCED CLASS QUESTIONS: 1. Why would sense organs be poorly developed in trematodes and cestodes? How would this differ if they were external parasites? Answer: Sense Organs in Trematodes and Cestodes: 1. Internal Parasitic Lifestyle: • Trematodes (flukes) and cestodes (tapeworms) are internal parasites that inhabit the digestive tract, blood vessels, or other tissues of their host. • Their internal environment is relatively stable, and they rely less on complex sensory structures for navigation or finding hosts. 2. Reduced Need for Sensory Organs: • As internal parasites, trematodes and cestodes have limited exposure to external stimuli. • Their hosts provide a constant environment with readily available nutrients, reducing the need for well-developed sense organs. 3. Simplification of Body Structure: • Trematodes and cestodes have undergone evolutionary simplification, leading to the loss or reduction of unnecessary structures, including sensory organs. • Their body plans are streamlined for efficient nutrient absorption and reproduction rather than sensory perception. External Parasitic Lifestyle: 1. Increased Need for Sensory Organs: • External parasites, such as ectoparasitic trematodes or cestodes, inhabit the external surfaces of their hosts. • They encounter a more dynamic environment with changing stimuli, requiring well-developed sensory structures for host detection, attachment, and feeding. 2. Complex Sensory Structures: • External parasites often possess specialized sensory organs, such as chemoreceptors, photoreceptors, and mechanoreceptors, to detect and respond to external stimuli. • Well-developed sensory structures aid in host finding, attachment, and feeding, enhancing the parasite's ability to survive and reproduce in its external environment. 2. How can flukes and bladderworms live intimately inside our bloodstream, ducts and tissue without being rejected by our immune system, or easily affected by drugs? If customs are not easily changed, how can a scientist break the chain of transmission? Answer: Survival Strategies of Flukes and Bladderworms: 1. Evolutionary Adaptations: • Flukes and bladderworms have evolved several strategies to evade the host immune system and resist the effects of drugs: • Immunomodulation: They can modulate the host immune response, suppressing immune reactions or altering host immune function to avoid detection. • Surface Antigen Variation: They may possess surface antigens that undergo rapid variation, making it difficult for the host immune system to recognize and target them effectively. 2. Enclosed Environment: • Flukes and bladderworms live in relatively enclosed environments within the host's bloodstream, ducts, or tissues, reducing exposure to immune cells and drugs. • Physical Barrier: The host's tissues provide a physical barrier that limits the access of immune cells and drugs to the parasites. 3. Slow Reproductive Rate: • Flukes and bladderworms often have slow reproductive rates, reducing their susceptibility to drugs that target rapidly dividing cells. • Encystment: Some parasites may undergo encystment, forming protective cysts that shield them from immune responses and drug effects. Breaking the Chain of Transmission: 1. Interrupting the Life Cycle: • Scientists can break the chain of transmission by targeting different stages of the parasite's life cycle: • Vector Control: Targeting the intermediate hosts or vectors involved in the transmission of the parasites can prevent their spread to humans. • Hygiene and Sanitation: Promoting good hygiene practices and improving sanitation can reduce the risk of contamination and infection with parasite eggs or larvae. 2. Developing Effective Treatments: • Research efforts aimed at developing new drugs and vaccines can help combat fluke and bladderworm infections: • Drug Development: Developing new drugs that target specific metabolic pathways or unique features of the parasites can improve treatment efficacy. • Vaccine Development: Creating vaccines that target key antigens or stages of the parasite's life cycle can enhance host immunity and reduce infection rates. 3. Health Education and Awareness: • Educating communities about the risks of parasite infection and promoting preventive measures can help reduce transmission: • Health Education: Providing information about the importance of proper hygiene, sanitation, and food preparation practices can empower individuals to protect themselves from parasite infections. • Community Engagement: Engaging communities in disease surveillance, control programs, and research efforts can foster collaboration and support in the fight against parasitic infections. 3. The beef tapeworm currently infests cattle, a mammal of more recent evolution. Did tapeworms only evolve as parasites after their modern hosts appeared? If not, what were they doing before then? Answer: Evolution of Tapeworms: 1. Ancient Origins: • Tapeworms belong to the class Cestoda and have ancient origins, predating the appearance of their modern mammalian hosts. • Fossil evidence suggests that tapeworms evolved as early as the Paleozoic era, long before the emergence of mammals. 2. Early Evolutionary History: • Before the appearance of their modern mammalian hosts, tapeworms likely parasitized early vertebrates, such as fish and amphibians. • They may have also parasitized invertebrates, serving as intermediate hosts in their life cycle. 3. Life Cycle Strategies: • Tapeworms have complex life cycles that often involve multiple hosts. • Before parasitizing mammals, tapeworms may have utilized different hosts and environmental conditions, adapting to diverse ecological niches over millions of years of evolution. 4. Co-evolution with Hosts: • The evolution of tapeworms is closely linked to the evolutionary history of their hosts. • As vertebrates diversified and evolved, tapeworms likely co-evolved with their hosts, adapting to new host species and ecological conditions. 4. Parasitic tapeworms, and indeed most successful parasites, are generally innocuous or do not harm their host much. Why would they evolve this direction, rather than become more virulent? Answer: Evolutionary Strategies of Parasitic Tapeworms: 1. Host Survival and Transmission: • Parasitic tapeworms have evolved to maintain a delicate balance between exploiting their host for nutrients and ensuring host survival for their own transmission. • Low Virulence: Highly virulent parasites risk killing their host prematurely, which can reduce their own chances of transmission to new hosts. • Chronic Infections: Tapeworms typically cause chronic, low-grade infections in their hosts, allowing them to persist for long periods without causing severe harm. 2. Transmission Efficiency: • Parasites that cause minimal harm to their hosts are more likely to be transmitted to new hosts, ensuring the survival and propagation of the parasite population. • Co-evolution with Hosts: Over time, parasitic tapeworms have co-evolved with their hosts, developing strategies to evade host immune responses and establish long-term, symbiotic relationships. 3. Resource Optimization: • Tapeworms have evolved to efficiently utilize host resources without causing excessive damage. • Energy Conservation: Low-virulence parasites conserve energy by avoiding host immune responses and maintaining a stable, long-term relationship with their host. 4. Evolutionary Trade-offs: • Highly virulent parasites may face evolutionary trade-offs, where increased virulence is accompanied by reduced transmission and fitness. • Optimal Balance: Parasitic tapeworms have evolved to strike an optimal balance between exploiting host resources and ensuring host survival and transmission, maximizing their own fitness over evolutionary time scales. CHAPTER 15 GNATHIFERANS AND SMALLER LOPHOTROCHOZOANS CHAPTER OUTLINE 15.1 Clade Polyzoa (Figure 15.1) A. The clade Polyzoa unites cycliophorans with entoprocts and ectoprocts. B. The close relationship between entoprocts and ectoprocts was originally proposed based on morphological characteristics, but phylogenetic studies support the association. C. Cycliophorans, which were discovered later, exhibit similarities with entoprocts. 15.2 Phylum Cycliophora (Figure 15.2) A. Characteristics 1. Live exclusively on the mouthparts of marine decapod crustaceans in the northern hemisphere. 2. Attach to bristles with an adhesive disc on the end of an acellular stalk. 3. Feed by collecting bacteria or bits of food dropped from their lobster host on a ring of compound cilia that surrounds the mouth. 4. Simple body plan: mouth leads to U-shaped gut ending with an anus that opens outside the ciliated ring. 5. Body is acoelomate. 6. Epidermis is cellular and surrounded by a cuticle. 7. Life cycle has both sexual and asexual phases. a. Feeding animals make internal buds called Pandora larvae, which become new feeding individuals upon release; clone members occupy vacant areas on the lobster mouthparts. b. In sexual reproduction a male larvae is released and settles atop another animal housing a female larvae. c. A male larvae produces secondary males with reproductive organs; internal fertilization occurs as secondary male mates with a female larva leaving the body of a feeding animal. d. Once fertilization occurs, a chordoid larva develops inside the body of its mother, consuming it. 15.3 Phylum Entoprocta A. Diversity (Figure 15.3) 1. About 150 species of Entoprocts occur worldwide, usually in marine environments. 2. They are less than 5 mm long and mostly microscopic, resembling hydroid cnidarians. 3. Urnatella gracilis is a common freshwater species in North America. B. Form and Function 1. The body or calyx is cup shaped and bears a circular crown of ciliated tentacles. 2. It attaches by a stalk with adhesive glands; tentacles and stalk are continuations of the body wall. 3. The body wall has a cuticle, cellular epidermis and longitudinal muscles 4. The 8–30 tentacles are ciliate on lateral and inner surfaces. 5. Tentacles can roll inward but cannot be retracted into the calyx. 6. The gut is U-shaped with both mouth and anus opening within the circle of tentacles, hence its phylum name. 7. Long cilia on the sides keep a current bringing in particles; short cilia on the inner surfaces capture food and direct it to the mouth. 8. A pair of protonephridia is embedded in the gelatinous parenchyma. 9. A well-developed nerve ganglion is on the ventral side of the stomach. 10. There are no circulatory or respiratory organs. 11. Some are monoecious, some dioecious and some first produce sperm and later eggs. 12. Fertilized eggs develop in a brood pouch between gonopore and anus. 13. Modified spiral cleavage leads to a trochophore-like larva. 15.4 Ectoprocts (Figures 15.4 − 15.6) 1. Characteristics a. Contains aquatic animals that often encrust hard surfaces. b. Most are sessile, some slide slowly, and others crawl actively across surfaces they inhabit. c. Mostly they are colony builders; each member is less than 0.5 mm. d. Colony members are called zooids. e. Zooids feed by extending their lophophores into surrounding water to collect tiny particles. f. Zooids secrete an exoskeleton which they live in. g. The exoskeleton (zooecium) may be gelatinous, chitinous, or stiffened with calcium and possibly impregnated with sand. h. Shape may be boxlike, vaselike, oval, or tubular. i. Are about 4500 living species. j. Live in both shallow freshwater and marine habitats. k. Some colonies form limy encrustations on seaweed, shells, and rocks; others form fuzzy or shrubby growths or erect branching colonies. l. Freshwater colonies may form mosslike colonies on stems of plants or on rocks. 2. Form and Function a. Each member lives in a tiny chamber called the zoecium. b. Each zooid consists of a feeding polypide and a case-forming cystid. c. A polypide includes the lophophore, digestive tract, muscles, and nerve centers. d. A cystid includes the body wall of an animal, together with its secreted exoskeleton. e. Polypides pop up to feed, but withdraw quickly at the slightest disturbance. 1) To extend its tentacular crown, muscles contract; this increases hydrostatic pressure within the body cavity and pushes the lophophore out. 2) Other muscles contract to withdraw the crown to safety. f. To feed, the lophophore is extended and the tentacles are spread out into a funnel. 1) Cilia on the tentacles draw water into the funnel. 2) Food particles caught by cilia in the funnel are drawn into the mouth by the pumping action of the muscular pharynx and by the action of the cilia. g. Digestion begins extracellularly in the stomach and is completed intracellularly within the intestine; the gut is complete. h. Respiratory, vascular, and excretory organs are absent. 1) Gas exchange is through body surface. 2) A ganglionic mass and a nerve ring surround the pharynx, but no sense organs are present. 3. Reproduction a. Most are hermaphroditic. b. Some species shed eggs into seawater, but most brood their eggs. 1) Brooding occurs within the coelom and some have an external chamber called an ovicell. c. Sometimes embryos proliferate asexually from the initial embryo in a process called polyembroyony. d. Cleavage is radial but mosaic. e. Larva of nonbrooding species have a functional gut and swim for a few months before settling. f. Larva of brooding species do not feed and settle after a brief free-swimming existence. 1) They attach to substratum by secretions from an adhesive sac, then metamorphose to adult form. g. New colonies begin from this single metamorphosed primary zooid, which is called an ancestrula. h. The ancestrula undergoes asexual budding to produce many zooids of a colony. i. Freshwater ectoprocts undergo budding that produces statoblasts which are hard, resistant capsules containing a mass of germinative cells. (Figure 15.7) 15.5 Clade Kryptrochozoa A. The taxon Trochozoa consists of animals with a trochophore larva. B. Within the Trochozoa, three taxa are united as Kryptrochozoa, characterized by a hidden trochophore larval stage. C. The Kryptrochozoa comprise the nemerteans and two other taxa that form the clade Brachiozoa. 15.6 Clade Brachiozoa A. The clade Brachiozoa unites brachiopods with phoronids. Both possess a lophophore. 15.7 Phylum Brachiopoda (Figure 15.8) A. Characteristics 1. Brachiopoda are also called lamp shells and are an ancient group. 2. Presently there are about 325 living species, but the fossil record indicates there were some 12,000 species living during the Paleozoic and Mesozoic seas. 3. Brachiopods are attached, bottom-dwelling, marine froms that inhabitat mostly shallow but all depths. 4. Externally they resemble bivalved mollusks. 5. Attach to a substrate either directly or by a fleshy stalk called a pedicel. (Figure 15.9) 6. Shell valves distinguish the two classes of brachiopods: a. Articulata have a connecting hinge with an interlocking tooth-and-socket arrangement. b. Inarticulata are held together by muscles. 7. Their body occupies only the posterior space between the valves. 8. Extensions of the body wall form mantle lobes that line and secrete the shell. 9. They have a large horseshoe-shaped lophophore in the anterior mantle. a. Ciliated tentacles are involved in feeding and respiration. 10. Food sources include organic detritus and some algae. 11. Most have separate sexes; fertilization is external. 12. Cleavage is radial; coelom and mesoderm formation in some is enterocoelic. 15.8 Phylum Phoronida (Figure 15.10) A. Characteristics 1. Contains ~20 species of small, wormlike animals. 2. Most live on the substrate of shallow coastal waters, especially in temperate seas. 3. Range in length from few millimeters to 30 cm. 4. Each worm secretes a leathery or chitinous tube in which it lies free, but never leaves. a. The tubes may be anchored singly or in a tangled mass on rocks, shells, or pilings or buried in sand. 5. They thrust out the tentacles on the lophophore for feeding but when disturbed withdraw into the tube. 6. A lophophore has two parallel ridges curved in a horseshoe shape. 7. Cilia in the tentacles direct water currents toward a groove between two ridges on the lophophore. 8. Plankton and detritus caught in this current become entangled in mucus and are carried by cilia to the mouth. 9. The anus lies dorsal to the mouth. 10. Cilia in the stomach of the U-shaped gut aid in food movement. 11. The body wall consists of cuticle, epidermis, and both longitudinal and circular muscles. a. The protocoel is present as a small cavity in the epistome. b. A septum separates the mesocoel and the metacoel. 12. Phoronids have an extensive system of contractile blood vessels in a functionally closed circulatory system period a. They have no heart. b. Blood contains hemoglobin within nucleated cells. 13. They have a pair of metanephridia. a. A nerve ring sends nerves to tentacles and the body wall; the system is diffuse and lacks distinct ganglion. 14. They are both monoecious and dioecious. a. At least two species reproduce asexually. b. Fertilization may be internal or external. c. Cleavage is radial. 15.9 Phylum Nemertea (Rhynchocoela) (Figures 15.11–15.14) A. Characteristics 1. Nemerteans are often called ribbon worms; an alternative phylum name is Rhynchocoela. 2. They have a long muscular tube, the proboscis. 3. There are over 1000 species; most are less than 20 cm long. 4. Their general body plan is similar to that of turbellarians. 5. The epidermis is ciliated and has many gland cells. 6. The excretory system has flames cells; several have rhabdites. 7. They are mostly dioecious. 8. The helmet-shaped pilidium larva has a ventral mouth but no anus, resembling flatworms and trochophore larvae of annelids and molluscs. 9. The adult has an anus, producing a complete digestive system that is more efficient. 10. They are the simplest animals with a blood-vascular system. 11. A few are found in moist soil and freshwater; however most are marine. 12. A few are commensals or parasites. 13. Form and Function a. Slender and fragile, longer ones are difficult to study in the laboratory. b. Amphiporus is a common example; it is 20–80 mm long. c. It is dorsoventrally flattened with rounded ends. d. The body wall is ciliated columnar cells and layers of circular and longitudinal muscles. e. Partly gelatinous parenchyma fills the space around organs. f. The anterior end has ocelli, a mouth and a separate opening of the proboscis. g. The proboscis is an eversible organ protruded from a rhynchocoel for defense and catching prey. h. The proboscis is everted by fluid pressure and retracted by muscles; it has a sharp-pointed stylet at the tip. 14. Locomotion a. Movement is by both musculature and cilia. b. Some glide on the substrate. c. Some use the proboscis to attach and draw the body forward. 15. Feeding and Digestion a. Nemerteans are carnivorous and eat dead or living prey. b. The slime-covered proboscis wraps around prey and the stylet pierces and holds prey until it is thrust into the mouth. c. It pours a neurotoxin, tetrodotoxin (the toxin in puffer fishes) on its prey. d. The complete digestive system has a dilated stomach and an intestine with lateral ceca. e. The tract is lined with ciliated epithelium and glandular cells in the esophagus. f. Food digested in the intestinal tube is absorbed into the blood-vascular system. 16. Circulation a. The blood-vascular system has a single dorsal vessel and two lateral vessels. b. Blood is colorless and contains nucleated corpuscles. c. Some have colored pigments with unknown functions. d. There is no heart; blood is moved by muscular walls of blood vessels and by body movements. 17. Excretion and Respiration a. Near the edge of the planarian is a lateral tube with branches and flame cells. b. Wastes picked up from parenchymal spaces by flame cells are carried out excretory ducts. c. Protonephridia are so closely associated with circulatory system that they are truly excretory rather than simply osmoregulatory in function as in flatworms. d. Respiration occurs through the body surface. 18. Nervous System a. The brain is composed of four fused ganglia, one pair dorsal and one pair ventral. b. Five longitudinal nerves extend backward from the brain. c. The proboscis, ocelli and other sense organs have nerves leading to the brain. d. Sense organs include tactile papillae, sensory pits and grooves, and probably auditory organs. 19. Reproduction and Development a. Amphiporus is dioecious. b. Gonads discharge eggs or sperm through short ducts; fertilization occurs in the water. c. As eggs are produced, other visceral organs degenerate. d. Cleavage is spiral and determinate; mesoderm is derived from both endoderm and ectoderm. e. The rhynchocoel develops from mesoderm but is not homologous to the coelom in other coelomate phyla. f. A pilidium larvae bears a dorsal spike of fused cilia and lateral lobes. 20. Regeneration a. Nemerteans can easily regenerate; some fragment themselves at certain seasons. b. The tail sections can regenerate a new proboscis within a short time. 21. Classification of Phylum Nemertea Class Enopla Class Anopla 22. Phylogeny of Nemertea a. There has been much debate about the phylogenetic position of nemerteans. b. Larval forms vary. c. The nemertean body plan is controversial. 1) Are they coelomate or acoelomate? Lecture Enrichment 1. Provide either specimens or slides to compare size, structure, habitats and relative complexity of the different invertebrates. A long balloon can be used to demonstrate a hydrostatic skeleton; it is a soft “tissue” providing the architectural base and fluid response. If a live horsehair worm becomes available, nothing surpasses handing a container around with one wriggling away inside. “Vinegar eels” are available from biological supply companies and can be placed in a petri dish on an overhead projector. 2. Try to keep the major themes running through the coverage of these nine phyla: syncytial tissues, cuticle, longitudinal muscles, etc. in the forefront. Compare the body structure of the flatworm and the roundworm, with emphasis on body layers, digestive tract and reproduction. 3. Most of the nematode parasitic diseases are tropical; utilize slides or other audiovisuals to illustrate the ravages of elephantiasis, fluke infestations, etc. Many cultural habits have evolved to circumvent ingestion of these parasites; describe or solicit the benefits of hygiene, plumbing, etc. noting that some depend upon a minimal level of affluence that is not available in poor regions of the world. Many students do not realize the extent they avoid suffering due to U.S. social customs and civil engineering. In the case of filariasis, mosquitoes carry the microscopic worms and mosquito control interrupts the cycle. 4. Recent issues of the Morbidity and Mortality Weekly Report from the CDC will provide up-to-date data and cases of helminth infections. 5. The proposal of a new phylum Cycliophora provides a “current event” view of the process of gaining acceptance for a major new taxon. Consider how this would be different if it was merely the addition of a new species, a new genus, a new family, etc. The 1995 Nature 378: 711–714 article contains a large line drawing that can easily illustrate this new organism for a class. Commentary/Lesson Plan Background: Students who grew up in the United States bring almost no everyday experience base to understand nematodes and related pseudocoelomates. International students are more likely to be aware of the parasitic forms. Some students may be aware of the cultural beliefs that help us avoid Trichinella infections and the affluence that minimizes other worm transmission. Misconceptions: Strictly speaking, syncytial tissues are a violation of the cell theory; students will not usually stop to consider this, nor the importance of such a variation as a feature indicating ancestral lineage. Ascaris is unusually large and no more represents the huge numbers of microscopic nematodes than a blue whale represents mammals; but this is an excellent opportunity to explain the shortcomings of calling any one species “representative.” Western students assume any disease has either a surgical or pharmaceutical cure; note the lack of such cures for many roundworm infestations. Schedule: The extent an instructor wishes to elaborate on life cycles and provide illustrations can extend this section by several class periods. HOUR 1 15.1. Protostomes A. Phylogeny B. Structure 15.2. Phylum Gnathostomulida A. Characteristics 15.3. Phylum Micrognathozoa A. Characteristics 15.4. Phylum Rotifera A. Diversity B. Form and Function C. Phylogeny HOUR 2 15.5. Phylum Acanthocephala A. Diversity 15.6. Phylum Cycliophora A. Characteristics B. Form and Function 15.7. Phylum Gastrotricha A. Characteristics 15.8. Phylum Entoprocta A. Diversity B. Form and Function C. Lophophorates D. Ectoprocts 15.9 Phylum Brachiopoda A. Characteristics 15.10. Phylum Phoronida A. Characteristics 15.11. Phylogeny ADVANCED CLASS QUESTIONS: 1. A “pseudo-” something is a false something, but the cavity is actually a cavity. Therefore why is it called a pseudocoelom? Answer: Reason for the Term "Pseudocoelom": 1. True vs. False Coelom: • A coelom is a body cavity that is completely lined with mesoderm. In true coelomates, such as annelids and chordates, the coelom is derived entirely from mesoderm. • In pseudocoelomates, such as nematodes, the body cavity is only partially lined with mesoderm, with the other side being lined with endoderm. 2. Historical Classification: • When nematodes were first classified, their body cavity was observed to be different from the true coelom found in annelids and chordates. • Although the cavity in nematodes is a true body cavity, it is termed "pseudocoelom" to distinguish it from the true coelom found in other animals. 3. Structural Differences: • The term "pseudocoelom" is used to indicate that while nematodes have a body cavity, it differs structurally and developmentally from the true coelom found in other animals. • The prefix "pseudo-" is used to denote that the body cavity in nematodes is similar to a coelom but is not derived entirely from mesoderm. 2. A set number of cells and cell divisions is an unusual feature in the animal kingdom but extends across several of the phyla discussed here. Why would an organism find this to be a positive adaptation? Must all features be explained in adaptation terms? Is there a correlation between eutely and having a cuticle? Answer: Benefits of Eutely and Correlation with a Cuticle: 1. Reproductive and Developmental Efficiency: • Eutely, a characteristic feature in some animal phyla such as Nematoda, describes the fixed number of cells and cell divisions in an organism. • Positive Adaptation: Eutely ensures that each individual in a population has the same number of cells, leading to consistent body size and reproductive output. • Efficient Development: A set number of cells and cell divisions can result in efficient development and reproduction, ensuring the production of a consistent number of offspring. 2. Resource Optimization: • Eutely may provide an evolutionary advantage by optimizing resource allocation and energy utilization. • Adaptation to Stable Environments: In stable environments with consistent resource availability, maintaining a fixed cell number can be advantageous for population stability and persistence. 3. Not All Features Explained in Adaptation Terms: • While many biological features can be explained in terms of adaptation and evolutionary advantage, not all features necessarily have adaptive significance. • Developmental Constraints: Some features, such as eutely, may be the result of developmental constraints or historical evolutionary processes rather than direct adaptation to environmental conditions. 4. Correlation with Cuticle: • In some pseudocoelomate phyla, such as Nematoda, eutely is often associated with the presence of a protective cuticle. • Structural Support: A cuticle provides structural support and protection, and its formation may be correlated with the fixed cell number characteristic of eutely. • Maintenance of Body Shape: The presence of a cuticle may help maintain the shape and integrity of the organism, contributing to the overall fitness and survival of the species. 3. If most of these “minor” phyla are found worldwide, why were many not discovered until the last century? Answer: Reasons for Late Discovery of "Minor" Phyla: 1. Small Size and Obscure Habitats: • Many "minor" phyla, such as Nematoda, Rotifera, and Tardigrada, consist of small-sized organisms that inhabit obscure or hard-to-reach habitats. • Microscopic Size: Some organisms within these phyla are microscopic, making them difficult to observe and study without advanced microscopy techniques. 2. Technological Limitations: • Until the development of advanced microscopy techniques and molecular biology tools, many "minor" phyla remained undiscovered or poorly understood. • Limited Sampling: Traditional methods of biological sampling often overlooked these small and cryptic organisms, leading to their late discovery. 3. Taxonomic Challenges: • Taxonomic classification of "minor" phyla can be challenging due to their unique anatomical features and evolutionary relationships. • Lack of Expertise: The specialized knowledge required to identify and classify organisms within these phyla may have contributed to their late discovery. 4. Ecological Research Focus: • Early ecological research primarily focused on larger, more conspicuous organisms, overlooking the ecological importance and diversity of "minor" phyla. • Shift in Research Focus: In recent decades, there has been an increased focus on biodiversity and microbial ecology, leading to the discovery and recognition of many "minor" phyla. 4. Some of the phyla covered here have flame cells, others have solenocytes and others lack protonephridia. How does this diversity argue for classifying them together or splitting the phyla apart? Answer: Diversity of Excretory Structures and Classification: 1. Flame Cells, Solenocytes, and Protonephridia: • Different phyla exhibit a variety of excretory structures: • Flame Cells: Found in Platyhelminthes (flatworms), these are specialized excretory cells that remove metabolic wastes from the body cavity. • Solenocytes: Present in Rotifera (rotifers), these are similar to flame cells and function in excretion and osmoregulation. • Protonephridia: Found in phyla such as Nematoda (roundworms), these are complex excretory structures with tubules and ciliated structures. 2. Arguments for Classification: • Shared Characteristics: Despite differences in excretory structures, these phyla share other morphological and developmental characteristics. • Evolutionary Relationships: Similarities in body plan, developmental patterns, and genetic data suggest that these phyla may share a common evolutionary ancestry. 3. Arguments for Splitting the Phyla: • Functional Differences: The presence of different excretory structures may indicate divergent evolutionary pathways and functional adaptations. • Ecological Niches: Differences in excretory structures may reflect adaptations to different ecological niches and lifestyles. • Phylogenetic Analysis: Molecular phylogenetic analyses may reveal significant genetic divergence between these phyla, supporting their classification as separate groups. 5. There is a wide range of reproductive modes both within and among the phyla in this chapter, from monoecious to dioecious to protandrous to traumatic insemination. How does this affect our view of reproductive modes as a classification trait? Answer: Variability in Reproductive Modes and Classification: 1. Reproductive Diversity: • Phyla discussed in this chapter exhibit a wide range of reproductive modes, including monoecious (hermaphroditic), dioecious (separate sexes), protandrous (sequential hermaphroditism), and traumatic insemination. • Examples: Platyhelminthes (flatworms) exhibit monoecious and traumatic insemination, while some annelids (e.g., earthworms) are monoecious and others (e.g., leeches) are dioecious. 2. Implications for Classification: • Challenges in Classification: The diversity of reproductive modes complicates traditional classification schemes based solely on reproductive traits. • Adaptive Significance: Reproductive modes are influenced by ecological, physiological, and behavioral factors, making them poor indicators of evolutionary relationships. 3. Evolutionary Considerations: • Convergent Evolution: Similar reproductive modes may evolve independently in distantly related taxa due to similar ecological pressures. • Phylogenetic Analysis: Molecular phylogenetic analyses provide a more reliable way to assess evolutionary relationships and classify organisms based on shared genetic ancestry. 6. Just look at how morphologically similar the entoprocts are to Obelia, etc. Why are they not grouped with the cnidaria too? Answer: Entoprocts and Cnidarians: 1. Morphological Similarities: • Entoprocts share some morphological similarities with cnidarians, such as colonial forms with tentacles surrounding a central mouth. • Example: Obelia, a colonial hydrozoan, shares superficial similarities with some entoprocts. 2. Differences in Body Plan: • Despite superficial similarities, entoprocts and cnidarians have significant differences in their body plan, developmental characteristics, and genetic makeup. • Entoprocts: Lack cnidocytes (stinging cells) and possess a U-shaped gut, distinguishing them from cnidarians. • Cnidarians: Possess cnidocytes and exhibit radial symmetry, whereas entoprocts exhibit bilateral symmetry. 3. Phylogenetic Analysis: • Molecular phylogenetic analyses based on genetic data indicate that entoprocts are more closely related to other lophotrochozoan phyla, such as Bryozoa and Phoronida, than to cnidarians. • Genetic Evidence: Shared genetic sequences support the classification of entoprocts within the Lophotrochozoa rather than with cnidarians. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214
