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This document contains Chapters 12 to 13 CHAPTER 12 SPONGES AND PLACOZOANS CHAPTER OUTLINE 12.1. Origins of Multicellularity A. Advantages 1. Nature’s experiments with larger organisms without cellular differentiation are limited. 2. Increasing the size of a cell causes problems of exchange; multicellularity avoids surface-to-mass problems. 3. Cell assemblages in sponges are distinct from other metazoans; molecular evidence demonstrates that sponges are phylogenetically grouped. 12.2. Origin of Animals (Metazoa) (Figure 11.1) A. Evolution of the Animals 1. Evolution of a eukaryotic cell was followed by diversification. a. Modern descendants include protozoa as well as multicellular plants and animals. b. The term “metazoan” is now synonymous with “animal”. 2. Metazoans fall within the opisthokont clade. (Figure 11.17) 3. Choanoflagellates are solitary or colonial aquatic eukaryotes. a. Each cell has a flagellum surrounded by a collar of microvilli. b. Beating the flagellum draws water into the collar where microvilli collect mostly bacteria. c. Most are sessile, but one species attaches to floating diatom colonies. d. They strongly resemble sponge feeding cells. e. There is much debate whether sponge choanocytes are an ancestor to choanoflagellates. 4. In one approach to metazoan origins, researchers hypothesize transitional forms between protozoan ancestors and simple metazoans. 5. By comparing the genomes or proteomes of simple metazoans like sponges with other animals, scientists can discover what cell transmitters or morphogens the first metazoans possessed. 6. Recent research has revealed that the sponge genome contains elements that code for regulatory pathways of more complex metazoans, including proteins involved in spatial patterning. 7. Some biologists hypothesize that modern sponges are less morphologically complex than their ancestors. 12.3. Phylum Porifera (Figures 12.1–12.4) A. General Features 1. Sessile sponges draw food and water into their body. 2. Porifera means “pore-bearing”; their sac-like bodies are perforated by many pores. 3. Sponges use flagellated “collar cells”, or choanocytes, to move water. 4. The sponge body is an efficient aquatic filter. 5. Most of the approximately 15,000 species of sponges are marine; a few live in brackish water and some 150 live in fresh water. 6. Marine sponges are found in all seas and at all depths; they vary greatly in size. 7. Many sponge species are brightly colored because of pigments in their dermal cells. 8. Although their embryos are free-swimming, adult sponges are always attached. 9. Some sponges appear radially symmetrical, but many are irregular. 10. Some stand erect, some are branched, and some are encrusting. 11. Growth patterns often depend on characteristics of the environment. 12. Many animals live as commensals or parasites in or on sponges; sponges also grow on a variety of other living organisms. 13. Few animals prey on sponges; sponges may have an elaborate skeletal structure and often have a noxious odor. 14. Sponges and the microorganisms that live in or on them produce a wide variety of bioactive chemicals; there is interest in sponge culturing as a source of pharmaceuticals. 15. The skeletal structure of a sponge can be fibrous and/or rigid period a. If present, the rigid skeleton consists of calcareous or siliceous spicules. b. The fibrous portion results from collagen fibrils in the intercellular matrix. c. One form of collagen, spongin, comes in several types. d. The composition of the spicules, along with their shape, forms the basis of sponge classification. 16. Sponges are ancient, with a fossil record dating back to the early Cambrian. 17. Living sponges traditionally have been assigned to three classes: Calcarea, Hexactinellida, and Demospongiae. a. Members of Calcarea typically have calcium carbonate spicules with one, three, or four rays. b. Hexactinellids are glass sponges with six-rayed siliceous spicules. c. Members of Demospongiae have siliceous spicules, spongin, or both. 18. Homoscleromorpha contains sponges that lack a skeleton or have siliceous spicules without axialfilament. B. Form and Function (Figure 12.5) 1. Body openings consist of small incurrent pores or dermal ostia. 2. Incurrent pores have an average diameter of 50 μm 3. Inside the body, water is directed past the choanocytes where food particles are collected. 4. Choanocytes or flagellated collar cells line some of the canals. a. They keep the current flowing by beating of flagella. b. They trap and phagocytize food particles passing by. 5. Sponges non-selectively consume food particles sized between 0.1 μm and 50 μm. 6. The smallest particles are taken into choanocytes by phagocytosis; protein molecules may be taken in by pinocytosis. 7. Two other cell types, pinacocytes and archaeocytes, play a role in sponge feeding. 8. Types of Canal Systems a. Asconoids: Flagellated Spongocoels (Figure 12.6) 1) Asconoids are simplest; they are small and tube-shaped. 2) Water enters a large cavity, the spongocoel, lined with choanocytes. 3) Choanocyte flagella pull water through. 4) Asconoids occur only in class Calcerea: Leucosolenia and Clathrina are examples. b. Syconoids: Flagellated Canals (Figure 12.7) 1) They resemble asconoids but are bigger with a thicker body wall. 2) The wall contains choanocyte-lined radial canals that empty into the spongocoel. 3) Water entering filters through tiny openings called prosopyles. 4) The spongocoel is lined with epithelial cells rather than choanocytes. 5) Food is digested by choanocytes. 6) Flagella force the water through internal pores called apopyles into the spongocoel and out the osculum. 7) They pass through an asconoid stage in development but do not form highly branched colonies. 8) The flagellated canals form by evagination of the body wall; this is developmental evidence of being derived from asconoid ancestors. 9) Classes Calcarea and Hexactinellida have species that are syconoid; the genus Sycon is an example. c. Leuconoids: Flagellated Chambers (Figure 12.8) 1) These are most complex and are larger with many oscula. 2) Clusters of flagellated chambers are filled from incurrent canals, and discharge to excurrent canals. 3) Most sponges are leuconoid; it is seen in most Calcarea and in all other classes. 4) The leuconoid system has evolved independently many times in sponges. 5) This system increases flagellated surfaces compared to volume; more collar cells can meet food demands. 6) Some large sponges can filter 1500 liters of water per day. 9. Types of Cells in the Sponge Body (Figures 12.9, 12.10) a. Sponge cells are arranged in a gelatinous matrix called mesohyl. b. The mesohyl is the connective “tissue” of sponges. c. The absence of true tissues or organs means that all fundamental processes must occur at the level of individual cells. d. The only visible activities of sponges are slight alterations in shape, local contraction, propagating contractions, and closing and opening of incurrent and excurrent pores. e. These movements are very slow; but still interesting in that they suggest a whole body response in organisms that lack organization above the cellular level. f. Apparently excitation spreads from cell to cell by an unknown mechanism. g. Choanocytes 1) These are oval cells with one end embedded in mesohyl. 2) The exposed end has a flagellum surrounded by a collar. 3) A collar is made of adjacent microvilli forming a fine filtering device to strain food. 4) Particles too large to enter the collar are trapped in mucous and moved to the choanocyte where they are phagocytized. 5) Food engulfed by choanocytes is passed to neighboring archaeocytes for digestion. 6) Choanocytes also have a role in sexual reproduction. h. Archaeocytes 1) These cells move about in the mesohyl. 2) They phagocytize particles in the pinacoderm. 3) They can differentiate into any other type of cell. 4) Those called sclerocytes secrete spicules. 5) Spongocytes secrete spongin. 6) Collencytes secrete fibrillar collagen. 7) Lophocytes secrete large quantities of collagen are morphologically distinguishable from collencytes. i. Pinacocytes 1) These cells form the pinacoderm; they are flat epithelial-like cells. 2) Pinacocytes are somewhat contractile. 3) Some are myocytes that help regulate flow of water. 10. Cell Independence: Regeneration and Somatic Embryogenesis (Figure 12.11) a. Sponges have a great ability to regenerate lost parts and repair injuries. b. A complete reorganization of the structure and function of participating cells or bits of tissue occurs in somatic embryogenesis. c. Much experimental work has been done in this field. d. The process of reorganization seems to differ in sponges of differing complexity. e. Regeneration following fragmentation is one means of asexual reproduction. f. Asexual reproduction can also occur by bud formation. g. External buds are small individuals that break off after attaining a certain size. h. Internal buds or gemmules are formed by archaeocytes that collect in mesohyl and are coated with tough spongin and spicules; they survive drought, freezing, etc. (Figure 12.11) 11. Sexual Reproduction (Figure 12.12) a. Most are monoecious with both male and female sex cells in one individual. b. Sperm sometimes arise from transformed choanocytes. c. In some Demospongiae and Calcarea, gametes develop from choanocytes; others derive them from archaeocytes. d. Sponges provide nourishment to the zygote until it is released as a ciliated larva. e. In some, when one sponge releases sperm, they enter the pores of another. f. Choanocytes phagocytize the sperm and transfer them to carrier cells that carry sperm through mesohyl to oocytes. g. Some release both sperm and oocytes into water. h. The free-swimming larva of sponges is a solid parenchymula i. Calcarea and some Demospongiae have strange development.. 1) A hollow stomoblastula develops with flagellated cells toward the interior. 2) The blastula then turns inside out (inversion). 3) Flagellated cells or micromeres of the larva are at the anterior end; larger non-flagellated macromeres are at the posterior end. 4) Macromeres overgrow the micromeres at metamorphosis during settlement. 5) Flagellated micromeres become choanocytes, archaeocytes and collencytes; nonflagellated cells give rise to pinacoderm and sclerocytes. C. Class Calcarea (Calcispongiae) (Figures 12.13, 12.5, 12.6) 1. These are calcareous sponges with spicules of calcium carbonate. 2. The spicules are straight or have three or four rays. 3. Most are small sponges with tubular or vase shapes. 4. Many are drab in color, but some are bright yellow, green, red, or lavender. 5. Leucosolenia and Sycon are marine shallow-water forms often studied in laboratories. 6. Asconoid, syconoid and leuconoid forms all occur. D. Class Hexactinellida (Hyalospongiae): Glass Sponges (Figures 12.13, 12.14) 1. These are glass sponges with six-rayed spicules of silica. 2. Nearly all are deep-sea forms; most are radially symmetrical. 3. Root spicules attach them to a substratum. 4. Syncytia are produced by fusion of many cells; a trabecular reticulum is a single, continous synctial tissue. 5. Trabecular reticulum is bilayered and contains cells such as choanoblasts and archeocytes. 6. Choanoblasts and other cells are connected to each other, and to the trabecular reticulum, by cytoplasmic bridges. 7. Choanoblasts are unusual cells that make two or more flagellated outgrowths called collar bodies, which beat to drive water flow. E. Class Demospongiae (Figure 12.13, 12.15) 1. This class contains 95% of living sponge species. 2. Spicules are siliceous but not six rayed; they may be absent or bound together by spongin. 3. All are leuconoid and all are marine except for Spongillidae, the freshwater sponges. 4. Freshwater sponges flourish in summer and die in late autumn, leaving gemmules. 5. Marine demosponges are highly varied in color and shape. 6. Bath sponges belong to a group that lacks siliceous spicules but have spongin skeletons. 7. Freshwater sponges are widely distributed in well-oxygenated ponds and streams. 8. Freshwater sponges are most common in midsummer. They reproduce sexually, but existing genotypes may also reappear annually from gemmules. F. Class Homoscleromorpha 1. Homoscleromorphs are often overlooked marine sponges because they live in cryptic habitats. 2. Sponges in this class possess unique features including a pinacoderm layer with a true basement membrane or ECM. a. Cells in this layer connect to each other with adherens cell junctions, making it a true tissue. 3. The Homoscleromorphs are divided into two clades, one without spicules and the other with spicules that do not form around a central longitudinal filament. G. Phylogeny and Adaptive Diversification 1. Phylogeny a. Sponges appeared before the Cambrian. b. Glass sponges rapidly expanded in the Devonian. c. One theory is that sponges arose from choanoflagellates; however, some corals and echinoderms also have collar cells, and sponges acquire them late in development. d. Molecular rRNA evidence suggests a common ancestor for choanoflagellates and metazoans and that sponges and Eumetazoa are sister groups with Porifera splitting off before radiates and placozoans. e. Classes of sponges are distinguished on the basis of spicule form and chemical composition. f. Phylogenetic studies indicate that sponges with calcareous spicules in the class Calcarea belong to a separate clade than those with spicules made of silica in the other classes. 2. Adaptive Diversification a. Porifera are a highly successful group; their diversification centers on their water-current system and its degree of complexity. b. Within the Demospongiae, a new feeding mode has evolved for a family of sponges found in deepwater caves. 1. They have a fine coating of tiny hook-like spicules over their body; this spicule layer entangles crustaceans. 2 .Later, filaments of the sponge body grow over the prey. 3. These sponges are carnivores, not suspension feeders. 4. They have siliceous spicules, but lack choanocytes and internal canals. 5. This illustrates the non-directional nature of evolution. H. Classification Class Calcarea Class Hexactinellida Class Demospongiae Class Homoscleromorpha 12.4. Phylum Placozoa A. Trichoplax adhaerens 1. This marine animal is the sole species of phylum Placozoa. 2. K. G. Grell described the phylum in 1971. 3. It has no symmetry, and no muscular or nervous organs. 4. Cell layers include: a dorsal epithelium, a thick ventral epithelium of monociliated cells and nonciliated gland cells; the space between the epithelia contain fibrous “cells” within a contractile syncytium. (Figure 12.17) 5. It glides over food, secretes digestive enzymes and absorbs the nutrients. 6. Grell considered it diploblastic: dorsal epithelium as ectoderm and ventral epithelium as endoderm. 7. Soon research will reveal the branching order (see cladogram inside front cover). Lecture Enrichment 1. Either provide specimens or show slides to compare size, structure, habitats and relative complexity of the different sponges. 2. This group was present before the sudden explosion of development of animal phyla during the early Cambrian period, based on fossil evidence. 3. Many students perceive evolution as a continuous development of more-and-more complexity. This is not true, especially for parasitic forms, and this is critical to understanding why some researchers theorize the primitive Mesozoa and Placozoans are derived from later phyla and lost size and structures. Commentary/Lesson Plan Background: Because these organisms are mostly microscopic or remote from inland temperate populations, few students will be directly familiar with any of these aside from specific targeted biology labwork. Coastal students may have some experience with marine sponges but none are likely to have seen freshwater sponges. Actual specimens are the only way to provide a hands-on feel of the sponge skeleton. Visuals will be important to illustrate the various structures. Misconceptions: Few students will appreciate the tremendous water filtering ability of sponges and may assume that since they are “primitive,” they must be unimportant. Schedule: HOUR 1 12.1. Origins of Multicellularity A. Advantages 12.2. Origin of Metazoa A. Three Theories of Unicellular Origin of Metazoans B. Molecular Evidence HOUR 2 12.3. Phylum Porifera A. General Features B. Form and Function C. Reproduction D. Class Calcarea (Calcispongiae) E. Class Hexactinellida (Hyalospongiae) F. Class Demospongiae G. Phylogeny and Adaptive Diversification H. Classification 12.4. Phylum Placozoa A. Trichoplax adhaerens ADVANCED CLASS QUESTIONS: 1. Explore the relationships between the Sponges and Placozoans by outlining their similarities and differences. Answer: Similarities between Sponges and Placozoans: 1. Simple Body Organization: • Both sponges and placozoans exhibit a simple body organization without true tissues or organs. • They lack specialized organs and organ systems, relying instead on simple cellular structures for essential functions. 2. Sessile Lifestyle: • Sponges and placozoans are predominantly sessile organisms, often attaching themselves to a substrate. • They remain in one location for most of their lives, relying on water currents for feeding and gas exchange. 3. Filter Feeding: • Both sponges and placozoans are filter feeders, capturing food particles from the surrounding water. • They use specialized cells or structures to filter and ingest organic particles and microscopic organisms. Differences between Sponges and Placozoans: 1. Body Structure: • Sponges have a porous body with a complex network of channels and chambers. • Placozoans have a simple, flattened body shape without any defined symmetry or internal structures. 2. Cell Types: • Sponges possess specialized cells, including choanocytes (collar cells), which are involved in feeding and generating water currents. • Placozoans have only two cell layers: an upper epithelial layer and a lower fiber-like layer, with no specialized cell types. 3. Reproduction: • Sponges reproduce both sexually and asexually, with some species capable of producing gemmules for asexual reproduction. • Placozoans reproduce primarily through asexual reproduction by binary fission, although sexual reproduction also occurs. 2. Discuss the relationship between the sponges and the flatworms, if any. Answer: The relationship between sponges and flatworms is not close, as they belong to different phyla (Porifera and Platyhelminthes, respectively) and exhibit significant differences in their body plans, lifestyles, and evolutionary histories. Differences between Sponges (Porifera) and Flatworms (Platyhelminthes): 1. Body Structure: • Sponges are multicellular organisms characterized by a porous body with a simple cellular organization. • Flatworms are bilaterally symmetrical organisms with three germ layers (ectoderm, mesoderm, and endoderm) and organ-level organization. 2. Feeding and Digestion: • Sponges are filter feeders, capturing food particles from water passing through their porous bodies. • Flatworms have a centralized digestive system with a mouth and gastrovascular cavity, allowing for more efficient digestion and nutrient absorption. 3. Reproduction: • Sponges reproduce both sexually and asexually, with some species capable of producing gemmules for asexual reproduction. • Flatworms exhibit various modes of reproduction, including sexual reproduction and asexual reproduction through regeneration or fission. 4. Tissue Organization: • Sponges lack true tissues and organs, with a cellular organization that includes specialized cells but no defined tissues. • Flatworms have organ-level organization, with distinct tissues and organ systems, including a nervous system and excretory system. Overall, sponges and flatworms are evolutionary distinct groups with few similarities and significant differences in their body plans, physiological processes, and reproductive strategies. 3. Explain why the Hexactinellida may someday be contained in a new phylum Answer: Hexactinellida, also known as glass sponges, exhibit unique characteristics that distinguish them from other sponge groups (Demospongiae and Calcarea). These distinctive features suggest that Hexactinellida may someday be contained in a new phylum. Some reasons for this include: 1. Siliceous Skeleton: • Hexactinellida sponges have a skeleton made of silica spicules, which are six-pointed, often fused, and made of hydrated silicon dioxide (glass). • The siliceous skeleton of Hexactinellida differs significantly from the calcareous or spongin skeletons found in other sponge groups. 2. Syncytial Tissues: • Hexactinellida sponges have syncytial tissues, where multiple cells share a single cytoplasmic mass and are interconnected by cytoplasmic bridges. • This syncytial organization is unique among sponges and more closely resembles the tissue organization found in some cnidarians. 3. Unique Reproductive Strategies: • Hexactinellida sponges exhibit unique reproductive strategies, including viviparity (live birth) and the production of large, yolk-rich eggs. • These reproductive characteristics are distinct from those of other sponge groups and suggest a separate evolutionary lineage. 4. Molecular Phylogenetics: • Molecular phylogenetic studies have suggested that Hexactinellida may represent a distinct evolutionary lineage within the Porifera. • Genetic analyses have revealed significant differences in the genetic makeup of Hexactinellida compared to other sponge groups, supporting the idea of a separate phylum. Overall, the unique combination of morphological, physiological, and genetic characteristics observed in Hexactinellida sponges suggests that they may eventually be classified within a new phylum separate from other sponge groups. CHAPTER 13 RADIATE ANIMALS CHAPTER OUTLINE 13.1. Phylum Cnidaria (Figure 13.1) A. Cnidarian Life History 1. Over 9,000 species are in the phylum Cnidaria. 2. Cnidaria have specialized cells (cnidocytes) that contain a specialized stinging organelle, the nematocyst. 3. Nematocysts are only formed and used by Cnidarians. 4. Fossil cnidarian specimens are dated to over 700 million years ago. 5. Today, they are most common in shallow marine environments, some are freshwater but none are terrestrial. 6. Some ctenophores, molluscs and flatworms eat hydroids and use the stinging nematocysts in their own defense. 7. Some live symbiotically; algae in reef-building corals are critical to coral reef formation. 8. The four classes of Cnidaria are Hydrozoa, Scyphozoa, Cubozoa and Anthozoa. (Figure 13.2) 9. A fifth class, Staurozoa, has been proposed. B. Characteristics of Cnidaria 1. All are aquatic and mostly marine. 2. Radial or biradial symmetry forms oral and aboral ends. 3. The two body types are the free-swimming medusae and the polyps. 4. They have a diploblastic body, with two layers: epidermis and gastrodermis. 5. Mesoglea is an extracellular matrix that lies between body layers. 6. An incomplete gut called the gastrovascular cavity. 7. Extracellular digestion in gastrovascular cavity and intracellular digestion in gastrodermal cells. 8. The muscular system has an outer layer of longitudinal fibers and an inner layer of circular fibers; contractions occur via epitheliomuscular cells. 9. Sense organs for balance (statocysts) and photosensitivity (ocelli). 10. Nerve net with symmetrical and assymetrical synapses; diffuse conduction; two nerve rings in hydrozoan medusae. 11. Asexual reproduction is by budding in polyps; some colonies form polymorphism. 12. Sexual reproduction by gametes in all medusae and some polyps; Monoecious or Dioecious; holoblastic indeterminate. 13. There is no excretory or respiratory system. 14. There is no coelomic cavity. C. Form and Function 1. Cnidaria have two basic body plans. 2. A polyp is a hydroid form. (Figure 13.3) a. Polyps are an adaptation to a sedentary life. b. The body is tubular with the mouth directed upward and surrounded by tentacles. c. The mouth leads into a blind gastrovascular cavity. d. The aboral end is attached to substrate by a pedal disc. e. Polyps may reproduce asexually by budding, fission, or pedal laceration. f. In colonial forms, the polyps may be specialized for feeding, reproduction or defense. g. In class Hydrozoa, feeding polyps (hydranths) can be distinguished from reproductive polyps (gonangia) by the absence of tentacles in gonangia. 3. A medusa is bell or umbrella shaped, and is usually free-swimming. (Figures 13.3, 13.10) a. The mouth is directed downward; tentacles may extend down from the rim of the umbrella. b. Medusae have statocysts and ocelli; Integration of sensory information into a motor response is the function of a nerve ring located at the base of the bell. c. Velum differentiates hydromedusae from scyphomedusae, a shelf-like fold of tissue from the bottom of the bell that extends into the bell. D. Life Cycles (Figure 13.7) 1. Polyps and medusae play different roles in the cnidarian life cycle. 2. Typically, the zygote develops into a motile planula larva. 3. The planula settles, and metamorphoses into a polyp. 4. Polyps may reproduce other polyps asexually, but eventually produces a free-swimming medusa by asexual reproduction. 5. Medusae are produced by the process of strobilation. 6. Medusae are dioecious, and reproduce sexually. 7. True jellyfish in the class Scyphozoa have conspicuous medusa stages and very small polyps. 8. Most colonial hydroids feature a polyp stage and a medusa stage. 9. Some hydrozoans like Velella and Physalia form floating colonies. (Figure 13.15) 10. In Hydra, the only stage is a small freshwater polyp. E. Body Wall (Figures 13.3-13.5) 1. The cnidarian body comprisess of an outer epidermis and an inner gastrodermis separated by mesoglea. 2. In Hydra, the epidermal layer contains epitheliomuscular, interstitial, gland, sensory, and nerve cells. 3. The mesoglea lies between the epidermis and the gastrodermis and is attached to both layers. a. It is gelatinous, and both epidermal and gastrodermal cells send processes into it. b. The mesoglea is continuous in polyps, extending over both body and tentacles. c. The mesoglea helps to support the body. d. In anthozoans, the mesoglea is thick and possesses amoeboid cells. e. It is also thick in schyphozoan medusae and contains amoeboid cells and fibers. f. The mesoglea is much thinner in hydromedusae and lacks cells. F. Cnidocytes (Figures 13.4, 13.6) 1. Many cnidarians are effectice predators. 2. This is made possible by the presence of a unique cell type, the cnidocyte. 3. Cnidoctyes are borne in invaginations of ectodermal cells, and in some gastrodermal cells. 4. A cnidocyte produces one of over 20 types of organelles known as cnidae that are discharged from the cell. 5. One type of cnida is the nematocyst. a. Nematocyts are tiny capsules made of chitin-like material and containing a coiled filament. b. A little lid or operculum covers the end of the capsule. c. The inside of the thread may have tiny barbs or spines. 6. Except in Anthozoa, a modified cilium called a cnidocil functions as a trigger. 7. Both small organic molecules and vibrations sensitize anthozoan cnidocytes. 8. After cnidae are discharged, its cnidocyte is absorbed and another develops. 9. Mechanism of Nematocyst Discharge (Figure 13.15) a. The cell can generate a high osmotic pressure of 140 atmospheres within the cnidocyte. b. The osmotic pressure falls as the hydrostatic pressure increases. c. When stimulated, the high internal osmotic pressure causes water to rush into the capsule. d. The operculum opens and rapidly releases the increased hydrostatic pressure launching the thread. e. At the everting end of the thread, the barbs point backward to anchor. f. Poison may be injected when it penetrates the prey. 10. Only a few jellyfish and the Portuguese man-of-war can seriously harm humans. (Figure 13.15) G. Feeding and Digestion (Figures 13.11, 13.19) 1. Polyps are typically carnivorous, catching prey with their tentacles and passing them to the gastrovascular cavity. 2. Inside the gastrovascular cavity, gland cells discharge enzymes to begin extracellular digestion. 3. Intracellular digestion continues in the gastrodermis. 4. The polyps in colonial hydrozoans pass food into a common gastrovascular cavity. 5. Feeding and digestion in hydromedusae is similar to that seen in the polyps. 6. Schyphomedusae have an extended mouth edge (manubrium) that is used in capturing and ingesting prey. 7. Anthozoan polyps are carnivorous; they can expand and stretch their tentacles in search of prey. 8. Some small anthozoans feed on minute forms captured by ciliary currents. 9. Corals supplement their nutrition with carbon collected from algal symbionts. H. Nerve Net 1. Two nerve nets, one at the base of epidermis and one at the base of gastrodermis, interconnect. 2. Nerve action potentials move across synapses by neurotransmitters. 3. Unlike higher animals, cnidarian nerves have neurotransmitters on both sides of the synapses allowing transmission either direction. 4. Cnidarian nerves lack the myelin sheath on axons. 5. Nerves synapse with both slender sensory cells and epitheliomuscular cells forming a neuromuscular system. 6. The nerve net pattern is also found in annelid, human and other digestive systems. 7. Cnidarians do not have a central nervous system, but some have argued that the nerve net and ring system function as one. 8. Rhopalia are groupings of nerves in sense organs and house chemoreceptors, statocysts, and ocelli; rhopalia are found in scyphomedusae and the medusa of cubozoans. I. Class Hydrozoa (Figures 13.1, 13.7─13.11) 1. Most Hydrozoa are marine and colonial with both polyp and medusa forms. 2. Hydra is not typical but is easy to study; the colonial Obelia is more exemplary. 3. A typical hydroid has a base, a stalk, and one or more terminal zooids. a. The base is a rootlike stolon, or hydrorhiza, giving rise to stalks called hydrocauli. b. The living part of the hydrocaulus is a tubular coenosarc. c. The hydrocaulus is covered by a chinous sheath, the perisarc. d. Individual zooids are attached to the hydrocaulus. e. Hydranths, or gastrozooids are feeding polyps with a single long tentacle. f. Gastrozooids may be thecate or athecate. g. Colonial hydroids bud off new individuals; the new individuals may be new feeding polyps or medusae. h. In Obelia, the medusae bud from a reproductive polyp called a gonangium. i. Hydroid medusae are usually smaller than schyphozoan medusae. j. The margin of the bell projects inward as a shelflike velum. k. The mouth opens at the end of a suspended manubrium. l. The mouth connects to a stomach and four radial canals. m. The radial canals connect to a ring canal that runs around the margin of the bell and connects n. with the hollow tentacles. o. The bell margin has many sensory cells. p. It typically also bears statocysts, specialized sense organs that function in equilibrium, and light-sensitive ocelli. 4. Investigations performed on Podocaryne carnea’s hydromedusae formation have revealed that a third cell layer, entoderm, differentiates into smooth and striated muscles. 5. Freshwater Medusae (Figure 13.12) a. Craspedacusta is a freshwater medusa that may have evolved from marine ancestors. 1) It is now found in Europe, the United States, and parts of Canada. 2) Craspedacusta medusae may reach a diameter of 20 mm; polyps are tiny (2 mm). 3) The polyp employs three methods of asexual reproduction. 6. Hydra: A Solitary Freshwater Hydrozoan (Figure 13.13) a. Hydra live on the underside of aquatic leaves and lily pads in clean fresh water. b. Hydra are found worldwide, with 16 species in North America. c. The body is a cylindrical tube; the aboral end has a basal or pedal disc for attachment. d. The mouth at the oral end is on a conical elevation called the hypostome. 1) A ring of 6–10 hollow tentacles encircles the mouth. 2) The mouth opens to a gastrovascular cavity. 3) Buds may project from the side, each develop a mouth and tentacles. e. Hydras feed on a variety of small crustaceans, insect larvae, and worms. 1) The mouth is located on a raised hypostome, and opens into the gastrovascular cavity. 2) Food organisms brushing against the extended tentacles are captured by nematocysts. 3) The captured organism is moved by the tentacles, and engulfed by the mouth. 4) Inside the gastrovascular cavity, gland cells discharge enzymes. f. Myofibrils in nutritive-muscular cells run at right angles to the body axis, forming a weak circular muscle layer. 1) Water enters gastrovascular cavity due to beating cilia, creating a hydrostatic skeleton. 2) Gastrodermal cells in green hydras bear symbiotic green algae cells. 3) Interstitial cells transform into other types when needed. 4) There are no cnidocytes in the gastrodermis. g. Epitheliomuscular cells form a covering and cause muscular contraction. (Figure 13.5) h. Undifferentiated interstitial cells can develop into cnidoblasts, sex cells, buds, or nerve cells, but not epitheliomuscular cells. i. Gland cells on adhesive disc secrete an adhesive and sometimes a gas bubble for floating period. j. Hydras have three types of cnidae. (Figure 13.4) 1) Penetrants penetrate prey and inject poison. 2) Volvents recoil and entangle prey. 3) Glutinants secrete an adhesive for locomotion and attachment. k. Sensory cells among epidermal cells bear a flagellum for chemical and tactile stimuli and synapse with nerve cells. l. Epidermal nerve cells are generally multipolar with both one-way and two-way synapses. . m. Epidermal nerve cells are generally multipolar with both one-way and two-way synapses. n. Hydras reproduce both sexually and asexually. 1) Asexual reproduction is by buds, which appear as outpocketings of the body wall. 2) Most hydras are dioecious. 3) Temporary gonads usually appear in autumn (Figure 13.14), stimulated by lower temperatures or stagnation. 4) Eggs and sperm are shed externally. 5) Zygotes undergo holoblastic cleavage to form a hollow blastula. 6) An encysted form endures the winter, then young hydras hatch in the spring. 7. Other Hydrozoans (Figure 13.15) a. Members of the orders Siphonophora and Chondrophora form polymorphic swimming or floating colonies. b. These floating colonies contain several types of polyp individuals. 1) Dactylozooids are fishing tentacles that sting prey and lift them to feeding polyps. 2) Gonophores are sacs containing ovaries or testes. 3) In Physalia, the float (pneumatophore) is hypothesized to have expanded from the original larval polyp. c. Other hydrozoans secrete calcareous skeletons resembling true corals and are the hydrocorals. (Figure 13.16) J. Class Scyphozoa (Figure 13.17) 1. Most of the larger jellyfishes belong to this class. 2. Nearly all float in open sea but one order is sessile, attached to seaweeds by a stalk. 3. Bells vary in shape and size; it is mostly mesogleal jelly, which is 95–96% water. 4. Unlike hydromedusae, the mesoglea contains ameboid cells and fibers. 5. Scyphozoans lack the shelf-like velum found in hydrozoan medusae. 6. The margin of the umbrella has indentations, each bearing a pair of lappets. 7. Between lappets is a sense organ, the club-shaped rhopalium bearing a hollow statocyst functioning in equilibrium. 8. The mouth is beneath the umbrella. 9. A manubrium forms four oral arms to capture and ingest prey. 10. Tentacles, manubrium, members of and the entire body may have nematocysts. 11. Aurelia has short tentacles; plankton caught in mucus of the umbrella are carried to food pockets. (Figure 13.18) 12. Cassiopeia and Rhizostoma lack umbrella tentacles but fold and fuse brachial canals that diversify the mouth. 13. Cassiopeia lies on its back forming currents to bring in plankton; symbiotic algal cells are in its tissues. 14. Extending from the stomach are four gastric pouches with gastric filaments covered with nematocysts; hydromedusae lack gastric filaments. 15. A complex system of radial canals branches out from pockets to a ring canal in the margin. 16. The nervous system consists of a nerve net; a subumbrellar net controls bell pulsations and a more diffuse net controls local reactions and feeding. 17. Sexes are separate and fertilization is internal in the gastric pouch of the female. 18. A zygote develops into a ciliated planula larva; this attaches and develops into a scyphistoma. 19. The scyphistoma undergoes strobilation to form buds, known as ephyrae, that break loose to form jellyfish medussae. (Figure 13.19) K. Class Staurozoa (Figure 13.20) 1. Commonly called stauromedusans. 2. These were previously considered unusual scyphozoans, even though their life cycle does not include the medusa stage. 3. It has a solitary polyp body that is stalked and uses an adhesive disk to attach to seaweeds, etc. 4. The top resembles a medusa with eight extensions (“arms”) ending in tentacle clusters. 5. Polyps reproduce sexually. L. Class Cubozoa (Figure 13.21) 1. These were once considered an order of Scyphozoa. 2. The medusoid form is dominant; the polyp is inconspicuous or unknown. 3. The umbrella is square; one or more tentacles extend from each corner. 4. At the base of each tentacle is a flat blade called a pedalium. 5. The umbrella edge turns inward to form a velarium, increasing swimming efficiency. 6. They are strong swimmers and feed mostly on fish in near shore areas. 7. The polyp stage is tiny; new polyps bud laterally, do not produce ephyrae but directly change into medusae. 8. The sea wasp is a potentially fatal cubomedusan from Australia. M. Class Anthozoa (Figures 13.22 − 13.24) 1. Anthozoans lack a medusa stage. 2. All anthozoans are marine, occur in both deep and shallow water, and vary in size. 3. There are three subclasses: Zoantharia, Cerianthpatharia and Alcyonaria. 4. Zoantharia and Cerianthpatharia are hexamerous; Alcyonaria are octomerous. (Figure 13.23) 5. Gastrovascular Cavity a. The cavity is large and partitioned by septa or mesenteries, inward extensions of body wall. b. Septa may be coupled or paired. (Figure 13.24) c. The mesoglea is mesochyme containing ameboid cells. d. There are no special organs for respiration or excretion. 6. Sea Anemones (Figures 13.22, 13.24) a. Actinaria polyps are larger and heavier than hydrozoan polyps. b. They attach to shells, rocks, timber, etc. by pedal discs; some burrow in mud or sand. c. A crown of tentacles surrounds the flat oral disc. (Figure 13.24) d. A slit-shaped mouth leads into a pharynx. e. The siphonoglyph is a ciliated groove that creates the water current into the pharynx. f. Currents carry in oxygen and remove wastes, and maintain fluid pressure for a hydrostatic skeleton. g. The gastrovascular cavity is divided into six pairs of primary septa or mesenteries. h. Smaller incomplete septa subdivide the large chambers and increase surface area. i. The free edge of each incomplete septum forms a septal filament with nematocysts and gland cells for digestion. j. Acontia threads at lower ends of septal filaments may protrude through the mouth to help secure prey. k. When in danger, water is rapidly expelled through pores as the anemone contracts to a small size. l. Feeding behavior responds to chemicals: asparagine activates feeding causing tentacles to bend toward the mouth and reduced glutathione induces swallowing. m. Longitudinal muscles of the epidermis only occur in the tentacles and oral disc. n. Longitudinal muscles are gastrodermal and located in the septa. o. Most anemones can glide slowly on pedal discs; some can swim with limited ability. p. Escape reactions occur in response to extracts from predators (e.g., sea stars, nudibranchs). (Figure 13.25) q. Most harbor symbiotic algae; some have a mutualistic relationship with hermit crabs. r. Some damselfishes shelter in sea anemones and have a skin mucus that protects them from triggering nematocysts. (Figure 13.26) s. Reproduction 1) Some have separate sexes and some are hermaphroditic. 2) Monoecious species are protandrous, producing sperm first and eggs later. 3) Gonads are on the margins of septa; fertilization is external or in the gastrovascular cavity. 4) The zygote becomes a ciliated larva. 5) In pedal laceration, small pieces of the pedal disc break off and regenerate a small anemone. 6) Longitudinal and transverse fission occur as well as budding. 7. Hexacorallian Corals (Figures 13.27–13.29) a. Members of the order Scleractinia are also called true or stony corals. b. They are miniature sea anemones that live in calcareous cups they have secreted. c. Their gastrovascular cavity is hexamerous but there is no siphonoglyph. d. Instead of a pedal disc, they secrete a limey skeletal cup with sclerosepta projecting up into the polyp. e. A sheet of living tissue forms over the coral surface, connecting all gastrovascular cavities. 8. Tube Anemones and Thorny Corals (Figures 13.30, 13.31) a. Members of subclass Ceriantipatharia have coupled but unpaired septa. b. Tube anemones are solitary and live in soft sediments. c. Thorny or black corals are colonial and attach to firm substrata. d. Both groups have few species and both live in warmer seas. 9. Octocorallian Corals (Figures 13.32–13.34) a. All have octomerous symmetry, eight pinnate tentacles and eight unpaired complete septa. b. All are colonial and gastrovascular cavities communicate through tubes called solenia. c. Solenia run through an extensive mesoglea (coenenchyme). d. Alcyonarian (Octocorallian) corals show great variation. 10. Coral Reefs a. Coral reefs have great productivity, rivaled only by tropical rainforests. b. Living plants and animals are limited to the top layer above the calcium carbonate deposits. c. Hermatypic corals and coralline algae form most coral reefs. (Figures 13.16B, 13.29) d. These corals require warmth, light, and the salinity of undiluted seawater, limiting them to waters between 30 degrees north and south. e. Microscopic zooxanthellae are photosynthetic and begin the food chain and recycle phosphorus and wastes. f. Coral bleaching threatens the symbosis between corals and zooxanthellae (Figure 13.35) g. Types of Reefs (Figure 13.35) 1) A fringing reef is near the land with no lagoon or a very narrow lagoon. 2) A barrier reef is parallel to shore with a wide and deep lagoon. 3) Atolls encircle a lagoon and have a steep bank on the seaward slope. 4) Patch or bank reefs are some distance back from any steep slopes. h. The side facing the sea is the reef front or fore reef slope. i. The reef crest is in shallow water or emergent at the top of the reef front; wave action breaks pieces off. j. The reef flat toward the shore receives this debris and coralline sand. k. These habitats support a diversity of corals and fish. l. Few nutrients enter or leave the system. m. Nutrients from fertilizer and sewage threaten coral reefs with excessive algal growth. n. Persian Gulf reefs have withstood surprising amounts of oil pollution. o. Coral reefs in many areas are threatened by factors mostly of human origin. p. Higher atmospheric concentrations of carbon dioxide (from burning hydrocarbon fuels) tends to acidify ocean water, which makes precipitation of CaCO3 by corals more difficult metabolically. N. Classification of Cnidaria Class Hydrozoa Class Scyphozoa Class Staurozoa Class Cubozoa Class Anthozoa Subclass Hexacorallia (Zoantharia) Subclass Ceriantipatharia Subclass Octocorrallia (Alcyonaria) 13.2. Phylum Ctenophora A. Ctenophore Life History 1. This phylum is composed of about 150 species. 2. All are marine species; most prefer warm waters. 3. Ctenophores have eight rows of comblike plates of cilia for locomotion. 4. Nearly all are free-swimming; only a few creep or are sessile. 5. They use the ciliated combs to propel themselves forward. 6. Body structure (Figure 13.37) a. Biradial symmetry resulting from two tentacles. b. Oral-aboral axis; no head. c. Translucent body with a gelatinous layer that contains muscle fibers; fibers are radial, meridional, and latitudinal bands. 7. Feeding Habits d. Trailing tentacles capture planktonic organisms by means of epidermal glue cells called colloblasts. (Figure 13.38C) e. Short tentacles collect food on the ciliated body surface. f. Ctenophores without tentacles feed on other gelatinous animals. 8. Ctenophores were previously divided between Tentaculata and Nuda classes. 9. Structuring classes within the Cnetophores is still being developed. B. Representative Type: Pleurobrachia (Figures 13.37, 13.38) 1. Pleurobrachia is a common example; it is transparent and 1.5–2 cm in diameter. 2. The oral pole bears the mouth opening; the aboral pole has the statocyst. 3. Comb Plates (Figure 13.38A) a. Eight equally-spaced bands called comb rows extend from the aboral to oral pole. b. Each band is made of transverse plates of long fused cilia called comb plates. c. The beat in each row begins at the aboral end and moves along combs to the oral end. d. All eight rows beat in unison; this drives the animal forward mouth-first. 4. Tentacles (Figure 13.37C) a. The two tentacles are long, solid and extensible. b. They retract into a pair of tentacle sheaths. c. The surface bears colloblasts or glue cells that secrete sticky material to hold animals. 5. The body wall resembles cnidarians with a gelatinous collenchyme in the interior. 6. In contrast to cnidarians, muscle cells are distinct and not part of the epitheliomuscular cells. 7. Digestion and Respiration (Figure 13.38) a. The gastrovascular system contains a mouth, pharynx, stomach and canals that run to the comb plates, tentacular sheaths, etc. b. Two blind canals terminate near the mouth. c. The aboral canal divides into two small anal canals that expel wastes. d. Digestion is both extracellular and intracellular. e. Respiration and excretion occur by diffusion across the body surface. 8. Nervous and Sensory Systems (Figures 13.38B, 13.38D) a. Their system resembles cnidarians; there is no central control. b. A subepidermal plexus is concentrated under each comb plate. c. The statocyst is a bell-like chamber; tufts of cilia sense changes in pressure from a statolith as the animal changes position. d. The epidermis bears sensory cells sensitive to chemical and other stimuli. e. When contacting an unfavorable stimulus, the cilia reverse their beat and it moves backward. f. Comb plates are sensitive to touch; they withdraw into the animal when touched. 9. Characteristics of Phylum Ctenophora a. Eight rows of combs (ctenes) arranged radially around body. b. Colloblasts present in most. c. Entirely marine. d. Symmetry biradial. e. Body ellipsoidal or spherical in shape; oral and aboral ends; no head. f. Adult body with gelatinous middle layer; diploblastic or triploblastic controversial. g. Complete gut. h. Extracellular digestion in pharynx. i. Two tentacles in most. j. Muscular contractions via muscle fibers. h. No respiratory system. i. No coelomic activity. 10. Reproduction and Development (Figure 13.39) a. Ctenophores are monoecious. b. Fertilized eggs are discharged through epidermis into the water. c. Cleavage is determinate; fate of blastomeres is determined early in contrast to cnidarians. d. The free-swimming cydippid larva somewhat resembles the adult. e. Some consider cellular nature of mesoglea to constitute a mesoderm. f. If the mesoderm is derived from endoderm, both ctenophores and cnidarians are diploblastic. 11. Other Ctenophores (Figure 13.40) a. Beroe is a large conical ctenophore that lacks tentacles. b. Venus’ girdle is a band-like ctenophore over one meter long. (Figure 13.40) c. Ctenoplana are flattened ctenophores that creep rather than swim. (Figure 13.40) d. Most ctenophores are bioluminescent at night. (Figure 13.37B) 13.3. Phylogeny and Adaptive Diversification A. Phylogeny of the Diploblasts 1. Distinctions between diploblastic and tripoblastic conditions are blurred because of recent detailed morphological studies and studies in gene expressions. 2. Cnetophores and cnidarians have the typical diploblastic characteristics; gelatinous middle surrounded by an outer epidermal layer derived from the ectoderm and by an inner gut lining derived from the endoderm. 3. However, the middle cells of the gelatinous layer are problematic. a. If these cells are derived from endoderm, then these organisms are characteristic of triploblasts; but these cells may be derived from the ectoderm. b. Some workers have referred to this layer as the ectomesoderm. c. Most cnidarians have few cells with the mesoglea, so there is little debate whether they are diploblastic. 4. However, during the hydrozoan medusae stage, the development of the entocodon layer has led to the suggestion that cnidarians are triploblastic. a. One of the products of the entocodon is striated muscle. b. Unlike contractile epitheliomuscular cells of other cnidarians, smooth and striated muscles are true muscle cells. c. In triploblasts, true muscles are produced by mesodermal cells, yet the hydrozoans entocodon is ectodermal in origin. d. Probes in gene expression found that homologous gene expression is occurring in those of triploblast mesoderm and diploblast mesoderm. e. Intepretation of these probes is not complete, but it may represent an independent origin of muscle in one branch of the diploblastic lineage. 5. Recent re-examination of the development of ctenophores has led to the observation that muscle cells in the middle layer originate from endodermal cells. 6. If this information is confirmed, then ctenophores are triploblastic along with the bilaterally symmetrical animals. 7. Body symmetry is also debated. a. An adult cntephore is radially symmetrical, however, the cnidarian planula larva swims with one end moving forward which could be designated as the anterior end and giving the planula a distinct anterior-posterior axis. b. The question remains: did the radially symmetrical cnidarians have a bilaterally symmetrical ancestor or does the genetic potential for bilateral symmetry predate the bilateral body plan? 8. Given the yet unanswered questions to origins of development, the branching order for the diploblastic phyla is not yet determined. 9. We predict a polytomy for cnidarian, ctenophoran, and placozoan branches. B. Cnidarian Phylogeny (Figure 13.2) 1. Relationships among cnidarian classes are still controversial: Which came first, the polyp or the medusa? a. One hypothesis postulates that the ancesteral cnidarian was a trachyline-like hydrozoan with a medusa stage. b. Another hypothesis suggests that the ancesteral cnidarian was an anthozoan polyp without a medusa in the life cycle. C. Adaptive Diversification 1. Evolution in both phyla has remained close in the basic structural plan. 2. Cnidarians have achieved large numbers of individuals and species, demonstrating large diversity considering the simplicity of their body plan. 3. They are efficient predators, some feeding on prey larger than themselves, and some feeding on small particles. 4. Some colonial forms grow to great size among corals and others show polymorphism and specialization of individuals within a colony. 5. Cnetophores adhere to the arrangement of comb plates and biradial symmetry but vary in body shape and presence or absence of tentacles. Lecture Enrichment 1. Provide specimens or slides to compare size, structure, habitats and relative complexity of the different invertebrates. 2. Discuss possible reasons why there appeared to be a sudden explosion of development of animal phyla during the early Cambrian period, based on the fossil record. Recent molecular work tracing backward from modern animals suggests that the animal lineage goes back over twice as far in time to a common animal ancestor. This would mean that two-thirds of the time period of animal evolution occurred before the Cambrian fossil record begins. The Ediacarian beds appear to have sizeable jellyfish fossils from the Precambrian; this is shown in the Attenborough Life on Earth series. 3. Although the nerve nets and statocyst sensory systems described here are simple, they allow for rather complex behaviors. This is a point to begin building student understanding of animal behavior as a cause-and-effect science rather than a mystical or vitalistic. 4. Darwin himself speculated on the formation of atolls. The formation of such coral islands has connections to the fields of geology, the history of nuclear testing, etc. Commentary/Lesson Plan Background: Because these organisms are mostly marine and require diving to experience, few inland students will be directly familiar with them aside from biology labwork and preserved specimens. There is a freshwater jellyfish found in deep lakes in the U.S. Coastal students may have some experience with marine Cnidaria and ctenophores. Visuals will be important for illustrating both diversity and structures. Misconceptions: A major misconception that extends to other early phyla is that all evolution and adaptation occurs only at the time the group arises. While the organisms are constrained by being in a lineage that did not develop more derived features of later groups, they nevertheless continue to adapt within their given architecture. Therefore many coral groups have been derived in more recent times (e.g. the symbiotic relationship of anemones with certain fish had to await the evolution of those fish). Schedule: HOUR 1 13.1. Phylum Cnidaria A. Cnidarian Life History B. Characteristics of Cnidaria C. Form and Function D. Nematocysts: Stinging Organelles E. Nerve Net F. Class Hydrozoa G. Class Scyphozoa H. Class Stauroza I. Class Cubozoa HOUR 2 J. Class Anthozoa K. Classification of Cnidaria L. Myxozoa 13.2. Phylum Ctenophora A. Ctenophore Life History B. Class Tentaculata C. Classification of Ctenophora 13.3. Phylogeny and Adaptive Diversification A. Phylogeny of Diploblasts B. Cnidarian Phylogeny C. Adaptive Diversification ADVANCED CLASS QUESTIONS: 1. How can a small soft hydra capture a larger Daphnia encased in a hard exoskeleton; that is, how do the nematocysts work? Answer: Nematocysts are specialized stinging organelles found in cnidarians such as Hydra. They enable Hydra to capture larger prey like Daphnia by the following mechanism: 1. Triggering Mechanism: • When a prey organism comes into contact with the tentacles of Hydra, it triggers the discharge of nematocysts. • Nematocysts are contained within specialized cells called cnidocytes, which are located on the tentacles of Hydra. 2. Barbed Harpoon Action: • The nematocyst is a coiled structure with a barbed harpoon inside. • Upon discharge, the nematocyst rapidly turns inside out, ejecting the harpoon-like structure into the prey organism. 3. Toxic Injection: • The harpoon injects a potent venom or toxin into the prey organism, paralyzing or killing it. • This allows Hydra to capture and consume larger prey, such as Daphnia, despite their hard exoskeletons. 2. How do other organisms manage to take over and incorporate nematocysts into their body tissues if these are distinct cnidarian organelles? Answer: Other organisms, such as some sea slugs and jellyfish predators, incorporate nematocysts into their own tissues for defense and predation by the following mechanism: 1. Ingestion of Cnidarians: • Organisms like sea slugs and certain fish species consume cnidarians, such as jellyfish or anemones, as part of their diet. • During ingestion, nematocysts may be released from the cnidarian's tissues. 2. Transport and Incorporation: • Some organisms are capable of transporting nematocysts from the cnidarian's tissues to their own. • Specialized cells within the predator's tissue, such as cnidocytes or cnidoblasts, can take up and incorporate nematocysts. 3. Defense and Predation: • Once incorporated, nematocysts can be used by the predator for defense against predators or for capturing prey. • These stolen nematocysts provide the predator with a means of delivering venom or toxins to deter predators or incapacitate prey. 3. You can put your hands safely inside a spongocoel but not a gastrovascular cavity; what is the difference? Answer: The spongocoel and the gastrovascular cavity are both internal cavities found in simple animals like sponges and cnidarians, respectively. The difference between them lies in their structure and function: 1. Spongocoel (in sponges): • The spongocoel is an internal cavity found within the body of a sponge. • It is lined with choanocytes (collar cells) that generate water currents and filter food particles. • The spongocoel acts as a central chamber for water circulation and food capture. • It is relatively safe to put hands inside a spongocoel as sponges lack specialized stinging cells or venomous structures. 2. Gastrovascular Cavity (in cnidarians): • The gastrovascular cavity is a central digestive chamber found in cnidarians such as Hydra and jellyfish. • It functions in both digestion and distribution of nutrients throughout the body. • The gastrovascular cavity is lined with gastrodermal cells that aid in digestion and nutrient absorption. • Unlike sponges, cnidarians possess specialized stinging cells called cnidocytes, which can discharge venomous nematocysts. • Putting hands inside a gastrovascular cavity can be dangerous due to the presence of nematocysts, which can cause painful stings and release toxins. 4. It is critical in humans that a nerve synapse only works in one direction; why is this not a problem for cnidarians? Answer: In humans, nerve synapses work in one direction due to the presence of specialized structures such as neurotransmitter receptors and synaptic vesicles, ensuring the unidirectional transmission of nerve impulses. However, in cnidarians like Hydra, unidirectional nerve transmission is not critical due to the following reasons: 1. Simple Nervous System: • Cnidarians have a relatively simple nerve net, lacking the complex organization found in vertebrates. • Nerve impulses in cnidarians can spread bidirectionally through the nerve net without the need for specialized structures to ensure unidirectional transmission. 2. Diffuse Nervous System: • The nervous system of cnidarians is diffuse and decentralized, with nerve cells (neurons) distributed throughout the body. • Nerve impulses can travel in multiple directions within the nerve net, allowing for rapid communication and coordination of responses. 3. Rapid Response: • Bidirectional nerve transmission allows for rapid responses to stimuli, such as prey capture or predator avoidance, in cnidarians. • While unidirectional nerve transmission is critical for precise control and coordination in complex nervous systems like those of vertebrates, bidirectional transmission is sufficient for the simpler nervous system of cnidarians. 5. Hydra are thin-bodied; anemones are thick-bodied. What is the structural difference? Answer: The structural difference between Hydra (thin-bodied) and anemones (thick-bodied) lies in their body shape and organization: 1. Hydra (Thin-Bodied): • Hydra have a tubular, elongated body shape with a thin, cylindrical body. • Their body consists of a single body column, with tentacles surrounding the mouth at one end. • Hydra have a relatively simple body organization, with a central gastrovascular cavity surrounded by outer cell layers. 2. Anemones (Thick-Bodied): • Anemones have a broader, more rounded body shape with a thick, muscular body. • Their body consists of a central column (the body) with a basal disc at the bottom for attachment. • Anemones have a more complex body organization compared to Hydra, with specialized structures such as mesenteries (radial partitions) and a larger gastrovascular cavity. 3. Structural Adaptations: • The thin-bodied structure of Hydra is adapted for a more streamlined and agile lifestyle, allowing for rapid movement and prey capture. • The thick-bodied structure of anemones provides increased surface area for prey capture, as well as greater stability and protection from predators. • The difference in body shape reflects the different ecological roles and lifestyles of Hydra and anemones within their respective habitats. 6. How can all of the free-swimming forms that lack any calcified endoskeleton or exoskeleton use muscles when they lack a base for muscle insertion? Answer: Free-swimming forms such as jellyfish (cnidarians) and comb jellies (ctenophores) lack a calcified endoskeleton or exoskeleton but still use muscles for movement. Despite lacking a solid skeletal structure, these organisms use the following mechanisms for muscle function: 1. Mesoglea as Muscle Attachment: • In cnidarians and ctenophores, muscles are attached to the mesoglea, a gelatinous, acellular layer between the outer epidermis and the inner gastrodermis. • Muscles in these organisms contract against the resistance provided by the elastic mesoglea, allowing for movement. 2. Hydrostatic Skeleton: • Cnidarians and ctenophores possess a hydrostatic skeleton, which consists of fluid-filled cavities surrounded by muscles. • Contraction of muscles against the fluid-filled cavities changes the shape of the body, producing movement. • Muscles in the body wall of these organisms exert pressure against the fluid-filled cavity, causing the body to change shape and move. 3. Radial Symmetry: • Many cnidarians, such as jellyfish, exhibit radial symmetry, allowing for coordinated movement in all directions. • Circular and longitudinal muscles in the body wall of these organisms contract and relax, allowing for swimming, feeding, and predator avoidance. 7. Why are some small jellyfish placed in Hydrozoa rather than Scyphozoa? Why are some fire corals placed in Hydrozoa rather than Anthozoa? Why are Cubozoa no longer placed in Scyphozoa? Answer: Some small jellyfish are placed in Hydrozoa rather than Scyphozoa, and some fire corals are placed in Hydrozoa rather than Anthozoa due to the following reasons: 1. Small Jellyfish in Hydrozoa: • Some small jellyfish, such as the genus Hydrozoa , are placed in the class Hydrozoa rather than Scyphozoa due to their life cycle and morphology. • These jellyfish exhibit a complex life cycle involving both polyp and medusa stages, typical of Hydrozoa. • Their medusa stage is small and often overlooked, resembling a tiny bell-shaped organism, characteristic of Hydrozoa. 2. Fire Corals in Hydrozoa: • Some fire corals, such as Millepora spp. , are placed in the class Hydrozoa rather than Anthozoa due to their colonial nature and cnidocyte arrangement. • Fire corals form colonies of interconnected polyps that secrete a calcareous skeleton, resembling hydrozoan colonies. • Additionally, the arrangement of cnidocytes in fire corals is more similar to that of hydrozoans than anthozoans. 3. Cubozoa Classification: • Cubozoa, commonly known as box jellyfish, were historically classified within the class Scyphozoa. • However, recent molecular and morphological studies have revealed significant differences between Cubozoa and traditional Scyphozoa. • Cubozoa have distinct features, including a box-shaped bell, complex eyes, and potent venom, warranting their classification as a separate class within Cnidaria. 8. Why are larval stages so critical to understanding early animal evolution? Answer: Larval stages are critical to understanding early animal evolution due to the following reasons: 1. Transitional Forms: • Larval stages represent transitional forms between ancestral and adult body plans, providing insights into evolutionary relationships and developmental pathways. • They often exhibit characteristics of ancestral forms that may have been lost or modified in adult organisms. 2. Dispersal and Colonization: • Larval stages facilitate dispersal and colonization of new habitats, allowing organisms to colonize new environments and adapt to changing ecological conditions. • Understanding larval dispersal patterns helps elucidate historical biogeography and the evolution of species distributions. 3. Evolutionary Innovation: • Larval stages play a crucial role in the evolution of novel features and developmental strategies. • Evolutionary changes in larval morphology, behavior, and life history traits can drive speciation and diversification over evolutionary time scales. 4. Conservation of Developmental Pathways: • Many ancestral developmental pathways are conserved in larval stages, providing clues to the evolutionary origins of complex body plans and developmental processes. • Comparative studies of larval development across different animal groups shed light on the genetic and developmental mechanisms underlying evolutionary change. 9. Outline the new thinking on the evolution of the Cnidaria and Ctenophora based on the planula-like larva. Answer: Recent studies on the evolution of Cnidaria and Ctenophora have proposed new hypotheses based on the planula-like larva, suggesting a closer evolutionary relationship between these two phyla. The new thinking includes the following points: 1. Shared Larval Morphology: • Both cnidarians and ctenophores possess a planula-like larva during their life cycle. • The planula-like larva is a ciliated, free-swimming stage that exhibits bilateral symmetry and a distinct apical organ. 2. Molecular Phylogenetics: • Molecular phylogenetic analyses have revealed similarities in the genetic makeup and developmental pathways between cnidarians and ctenophores. • Shared genetic markers and developmental genes suggest a closer evolutionary relationship between these two phyla than previously thought. 3. Evolutionary Scenario: • A new evolutionary scenario proposes that cnidarians and ctenophores diverged from a common ancestor that possessed a planula-like larva. • The planula-like larva represents a shared ancestral feature that has been retained in both phyla throughout evolutionary history. 4. Planula Hypothesis: • The Planula Hypothesis suggests that the planula-like larva is a synapomorphic trait of Cnidaria and Ctenophora, supporting their monophyletic origin. • This hypothesis challenges previous views of the evolutionary relationship between cnidarians and ctenophores, suggesting a closer evolutionary affinity between these two phyla. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214
