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Chapter 33

BIOL 1030 Chapter 33: Chapter 33 Invertebrates

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Biological Sciences
BIOL 1030
Scott Kevin

Chapter 33 Invertebrates Lecture Outline Overview: Life Without a Backbone • Invertebrates—animals without a backbone—account for 95% of known animal species and all but one of the roughly 35 animal phyla that have been described. • More than a million extant species of animals are known, and at least as many more will probably be identified by future biologists. • Invertebrates inhabit nearly all environments on Earth, from the scalding water of deep-sea hydrothermal vents to the rocky, frozen ground of Antarctica. Concept 33.1 Sponges are sessile and have a porous body and choanocytes • Sponges (phylum Porifera) are so sedentary that they were mistaken for plants by the early Greeks. • Living in freshwater and marine environments, sponges are suspension feeders. • The body of a simple sponge resembles a sac perforated with holes. • Water is drawn through the pores into a central cavity, the spongocoel, and flows out through a larger opening, the osculum. • More complex sponges have folded body walls, and many contain branched water canals and several oscula. • Sponges range in height from about a few mm to 2 m and most are marine. • About 100 species live in fresh water. • Unlike eumetazoa, sponges lack true issues, groups of similar cells that form a functional unit. • The germ layers of sponges are loose federations of cells, which are not really tissues because the cells are relatively unspecialized. • The sponge body does contain different cell types. • Sponges collect food particles from water passing through food- trapping equipment. • Flagellated choanocytes, or collar cells, lining the spongocoel (internal water chambers) create a flow of water through the sponge with their flagella and trap food with their collars. • Based on both molecular evidence and the morphology of their choanocytes, sponges evolved from a colonial choanoflagellate ancestor. • The body of a sponge consists of two cell layers separated by a gelatinous region, the mesohyl. • Wandering though the mesohyl are amoebocytes. • They take up food from water and from choanocytes, digest it, and carry nutrients to other cells. • They also secrete tough skeletal fibers within the mesohyl. • In some groups of sponges, these fibers are sharp spicules of calcium carbonate or silica. • Other sponges produce more flexible fibers from a collagen protein called spongin. • ? We use these pliant, honeycombed skeletons as bath sponges. • Most sponges are sequential hermaphrodites, with each individual producing both sperm and eggs in sequence. • Gametes arise from choanocytes or amoebocytes. • The eggs are retained, but sperm are carried out the osculum by the water current. • Sperm are drawn into neighboring individuals and fertilize eggs in the mesohyl. • The zygotes develop into flagellated, swimming larvae that disperse from the parent. • When a larva finds a suitable substratum, it develops into a sessile adult. • Sponges produce a variety of antibiotics and other defensive compounds. • Researchers are now isolating these compounds, which may be useful in fighting human disease. Concept 33.2 Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes • All animals except sponges belong to the Eumetazoa, the animals with true tissues. • The cnidarians (hydras, jellies, sea anemones, and coral animals) have a relatively simple body construction. • They are a diverse group with more than 10,000 living species, most of which are marine. • They exhibit a relatively simple, diploblastic body plan that arose 570 million years ago. • The basic cnidarian body plan is a sac with a central digestive compartment, the gastrovascular cavity. • A single opening to this cavity functions as both mouth and anus. • This basic body plan has two variations: the sessile polyp and the floating medusa. • The cylindrical polyps, such as hydras and sea anemones, adhere to the substratum by the aboral end and extend their tentacles, waiting for prey. • Medusas (also called jellies) are flattened, mouth-down versions of polyps that move by drifting passively and by contracting their bell- shaped bodies. • The tentacles of a jelly dangle from the oral surface. • Some cnidarians exist only as polyps. • Others exist only as medusas. • Still others pass sequentially through both a medusa stage and a polyp stage in their life cycle. • Cnidarians are carnivores that use tentacles arranged in a ring around the mouth to capture prey and push the food into the gastrovascular chamber for digestion. • Batteries of cnidocytes on the tentacles defend the animal or capture prey. • Organelles called cnidae evert a thread that can inject poison into the prey, or stick to or entangle the target. • Cnidae called nematocysts are stinging capsules. • Muscles and nerves exist in their simplest forms in cnidarians. • Cells of the epidermis and gastrodermis have bundles of microfilaments arranged into contractile fibers. • True muscle tissue appears first in triploblastic animals. • When the animal closes its mouth, the gastrovascular cavity acts as a hydrostatic skeleton against which the contractile cells can work. • Movements are controlled by a noncentralized nerve net associated with simple sensory receptors that are distributed radially around the body. • The phylum Cnidaria is divided into four major classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa. • The four cnidarian classes show variations on the same body theme of polyp and medusa. • Most hydrozoans alternate polyp and medusa forms, as in the life cycle of Obelia. • The polyp stage, often a colony of interconnected polyps, is more conspicuous than the medusa. • Hydras, among the few freshwater cnidarians, are unusual members of the class Hydrozoa in that they exist only in the polyp form. • When environmental conditions are favorable, a hydra reproduces asexually by budding, the formation of outgrowths that pinch off from the parent to live independently. • When environmental conditions deteriorate, hydras form resistant zygotes that remain dormant until conditions improve. • The medusa generally prevails in the life cycle of class Scyphozoa. • The medusae of most species live among the plankton as jellies. • Most coastal scyphozoans go through small polyp stages during their life cycle. • Jellies that live in the open ocean generally lack the sessile polyp. • Cubozoans have a box-shaped medusa stage. • They can be distinguished from scyphozoans in other significant ways, such as having complex eyes in the fringe of the medusae. • Cubozoans, which generally live in tropical oceans, are often equipped with highly toxic cnidocytes. • Sea anemones and corals belong to the class Anthozoa. • They occur only as polyps. • Coral animals live as solitary or colonial forms and secrete a hard external skeleton of calcium carbonate. • Each polyp generation builds on the skeletal remains of earlier generations to form skeletons that we call coral. • In tropical seas, coral reefs provide habitat for a great diversity of invertebrates and fishes. • Coral reefs in many parts of the world are currently being destroyed by human activity. • Pollution, overfishing, and global warming are contributing to their demise. Concept 33.3 Most animals have bilateral symmetry • The vast majority of animal species belong to the clade Bilateria, which consists of animals with bilateral symmetry and triploblastic development. • Most bilaterians are also coelomates. • The most recent common ancestor of living bilaterians probably lived in the later Proterozoic. • During the Cambrian explosion, most major groups of bilaterians emerged. Phylum Platyhelminthes: Flatworms are acoelomates with gastrovascular cavities. • Flatworms live in marine, freshwater, and damp terrestrial habitats. • They also include many parasitic species, such as the flukes and tapeworms. • Flatworms have thin bodies, ranging in size from nearly microscopic to tapeworms more than 20 m long. • Flatworms and other bilaterians are triploblastic, with a middle embryonic tissue layer, a mesoderm, which contributes to more complex organs and organ systems and to true muscle tissue. • While flatworms are structurally more complex than cnidarians, they are simpler than other bilaterians. • Like cnidarians, flatworms have a gastrovascular cavity with only one opening (and tapeworms lack a digestive system entirely and absorb nutrients across their body surface). • Unlike other bilaterians, flatworms lack a coelom. • The flat shape of a flatworm places all cells close to the surrounding water, enabling gas exchange and the elimination of nitrogenous wastes (ammonia) by diffusion across the body surface. • Flatworms have no specialized organs for gas exchange and circulation, and their relatively simple excretory apparatus functions mainly to maintain osmotic balance. • This apparatus consists of ciliated cells called flame bulbs that waft fluid through branched ducts that open to the outside. • Flatworms are divided into four classes: Turbellaria, Monogenia, Trematoda, and Cestoidea. • Turbellarians are nearly all free-living (nonparasitic) and most are marine. • Planarians, members of the genus Dugesia, are carnivores or scavengers in unpolluted ponds and streams. • Planarians move using cilia on the ventral epidermis, gliding along a film of mucus they secrete. • Some turbellarians use muscles for undulatory swimming. • A planarian has a head with a pair of eyespots to detect light, and lateral flaps that function mainly for smell. • The planarian nervous system is more complex and centralized than the nerve net of cnidarians. • Planarians can learn to modify their responses to stimuli. • Planarians reproduce asexually through regeneration. • The parent constricts in the middle, and each half regenerates the missing end. • Planarians can also reproduce sexually. • These hermaphrodites cross-fertilize. • The monogeneans (class Monogenea) and the trematodes (class Trematoda) live as parasites in or on other animals. • Many have suckers for attachment to their host. • A tough covering protects the parasites. • Reproductive organs nearly fill the interior of these worms. • Trematodes parasitize a wide range of hosts, and most species have complex life cycles with alternation of sexual and asexual stages. • Many require an intermediate host in which the larvae develop before infecting the final hosts (usually a vertebrate) where the adult worm lives. • The blood fluke Schistosoma infects 200 million people, leading to body pains and dysentery. • The intermediate host for Schistosoma is a snail. • Living within different hosts puts demands on trematodes that free-living animals do not face. • A blood fluke must evade the immune systems of two very different hosts. • By mimicking their host’s surface proteins, blood flukes create a partial immunological camouflage. • They also release molecules that manipulate the host’s immune system. • These defenses are so effective that individual flukes can survive in a human host for more than 40 years. • Most monogeneans are external parasites of fishes. • Their life cycles are simple, with a ciliated, free-living larva that starts an infection on a host. • While traditionally aligned with trematodes, some structural and chemical evidence suggests that they are more closely related to tapeworms. • Tapeworms (class Cestoidea) are also parasitic. • The adults live mostly in vertebrates, including humans. • Suckers and hooks on the head, or scolex, anchor the worm in the digestive tract of the host. • Tapeworms lack a gastrovascular cavity and absorb food particles from their hosts. • A long series of proglottids, sacs of sex organs, lie posterior to the scolex. • Mature proglottids, loaded with thousands of eggs, are released from the posterior end of the tapeworm and leave with the host’s feces. • In one type of cycle, tapeworm eggs in contaminated food or water are ingested by intermediary hosts, such as pigs or cattle. • The eggs develop into larvae that encyst in the muscles of their host. • Humans acquire the larvae by eating undercooked meat contaminated with cysts. • The larvae develop into mature adults within the human. Phylum Rotifera: Rotifers are pseudocoelomates with jaws, crowns of cilia, and complete digestive tracts. • Rotifers are tiny animals (5 µm to 2 mm), most of which live in freshwater. • Some live in the sea or in damp soil. • Rotifers are smaller than many protists but are truly multicellular, with specialized organ systems. • Rotifers have an alimentary canal, a digestive tract with a separate mouth and anus. • Internal organs lie in the pseudocoelom, a body cavity that is not completely lined with mesoderm. • The fluid in the pseudocoelom serves as a hydrostatic skeleton. • Through the movements of nutrients and wastes dissolved in the coelomic fluid, the pseudocoelom also functions as a circulatory system. • The word rotifer, “wheel-bearer,” refers to the crown of cilia that draws a vortex of water into the mouth. • Food particles drawn in by the cilia are captured by the jaws (trophi) in the pharynx and ground up. • Some rotifers exist only as females that produce more females from unfertilized eggs, a type of parthenogenesis. • Other species produce two types of eggs that develop by parthenogenesis. • One type forms females, and the other forms degenerate males that survive just long enough to fertilize eggs. • The zygote forms a resistant stage that can withstand environmental extremes until conditions improve. • The zygote then begins a new female generation that reproduces by parthenogenesis until conditions become unfavorable again. • It is puzzling that so many rotifers survive without males. • The vast majority of animals and plants reproduce sexually at least some of the time, and sexual reproduction has certain advantages over asexual reproduction. • For example, species that reproduce asexually tend to accumulate harmful mutations in their genomes faster than sexually reproducing species. • As a result, asexual species experience higher rates of extinction and lower rates of speciation. • A class of asexual rotifers called Bdelloidea consists of 360 species that all reproduce by parthenogenesis without males. • Thirty-five-million-year-old bdelloid rotifers have been found preserved in amber. • The morphology of these fossils resembles the female form. • DNA comparisons of bdelloids with their closest sexually reproducing rotifer relatives suggest that bdelloids have been asexual for far more than 35 million years. • Bdelloid rotifers raise interesting questions about the evolution of sex. The lophophorate phyla: ectoprocts, phoronids, and brachiopods are coelomates with ciliated tentacles around their mouths. • Bilaterians in three phyla—Ectoprocta, Phoronida, and Brachiopoda—are traditionally called lophophorate animals because they all have a lophophore. • The lophophore is a horseshoe-shaped or circular fold of the body wall bearing ciliated tentacles that surround and draw water toward the mouth. • The tentacles trap suspended food particles. • In addition to the lophophore, these three phyla share a U-shaped digestive tract and the absence of a head. • These may be adaptations to a sessile existence. • In contrast to flatworms, which lack a body cavity, and rotifers, which have a pseudocoelom, lophophorates have true coeloms completely lined with mesoderm. • Ectoprocts are colonial animals that superficially resemble plants. • In most species, the colony is encased in a hard exoskeleton. • The lophophores extend through pores in the exoskeleton. • Most ectoprocts are marine, where they are widespread and numerous sessile animals, with several species that can be important reef builders. • Ectoprocts also live in lakes and rivers. • Phoronids are tube-dwelling marine worms ranging from 1 mm to 50 cm in length. • Some live buried in the sand within chitinous tubes. • They extend the lophophore from the tube when feeding and pull it back in when threatened. • Brachiopods, or lampshells, superficially resemble clams and other bivalve molluscs. • However, the two halves of the brachiopod are dorsal and ventral to the animal, rather than lateral as in clams. • All brachiopods are marine. • Most live attached to the substratum by a stalk, opening their shell slightly to allow water to flow over the lophophore. • The living brachiopods are remnants of a richer past. • Thirty thousand species of brachiopod fossils have been described from the Paleozoic and Mesozoic eras. Phylum Nemertea: Proboscis worms are named for their prey- capturing apparatus. • The members of the Phylum Nemertea, proboscis worms or ribbon worms, have bodies much like those of flatworms. • However, they have a small fluid-filled sac that may be a reduced version of a true coelom. • The sac and fluid hydraulics operate an extensible proboscis, which the worm uses to capture prey. • Nemerteans range in length from less than 1 mm to several meters. • Nearly all nemerteans are marine, but a few species inhabit fresh water or damp soil. • Some are active swimmers, and others burrow into the sand. • Nemerteans and flatworms have similar excretory, sensory, and nervous systems. • However, nemerteans have an alimentary canal and a closed circulatory system in which the blood is contained in vessels. • Nemerteans have no heart, and the blood is propelled by muscles squeezing the vessels. Concept 33.4 Molluscs have a muscular foot, a visceral mass, and a mantle • The phylum Mollusca includes many diverse forms, including snails and slugs, oysters and clams, and octopuses and squids. • Most molluscs are marine, though some inhabit fresh water, and some snails and slugs live on land. • Molluscs are soft-bodied animals, but most are protected by a hard shell of calcium carbonate. • Slugs, squids, and octopuses have reduced or lost their shells completely during their evolution. • Despite their apparent differences, all molluscs have a similar body plan with a muscular foot (typically for locomotion), a visceral mass with most of the internal organs, and a mantle. • The mantle, which secretes the shell, drapes over the visceral mass and creates a water-filled chamber, the mantle cavity, with gills, anus, and excretory pores. • Many molluscs feed by using a straplike rasping organ, a radula, to scrape up food. • Most molluscs have separate sexes, with gonads located in the visceral mass. • However, many snails are hermaphrodites. • The life cycle of many marine molluscs includes a ciliated larva, the trochophore. • This larva is also found in marine annelids (segmented worms) and some other lophotrochozoans. • The basic molluscan body plan has evolved in various ways in the eight classes of the phylum. • The four most prominent are the Polyplacophora (chitons), Gastropoda (snails and slugs), Bivalvia (clams, oysters, and other bivalves), and Cephalopoda (squids, octopuses, cuttlefish, and chambered nautiluses). • Chitons are marine animals with oval shapes and shells divided into eight dorsal plates. • The chiton body is unsegmented. • Chitons use their muscular foot to grip the rocky substrate tightly and to creep slowly over the rock surface. • Chitons are grazers that use their radulas to scrape and ingest algae. • Almost three-quarters of all living species of molluscs are gastropods. • Most gastropods are marine, but there are also many freshwater species. • Garden snails and slugs have adapted to land. • During embryonic development, gastropods undergo torsion in which the visceral mass is rotated up to 180 degrees, so the anus and mantle cavity are above the head in adults. • After torsion, some of the organs that were bilateral are reduced or lost on one side of the body. • Most gastropods are protected by single, spiral shells into which the animals can retreat if threatened. • Torsion and formation of the coiled shell are independent developmental processes. • While gastropod shells are typically conical, those of abalones and limpets are somewhat flattened. • Many gastropods have distinct heads with eyes at the tips of tentacles. • They move by a rippling motion of their foot or by means of cilia. • Most gastropods use their radula to graze on algae or plant material. • Some species are predators. • In these species, the radula is modified to bore holes in the shells of other organisms or to tear apart tough animal tissues. • In the tropical marine cone snails, teeth on the radula form separate poison darts, which penetrate and stun their prey, including fishes. • In place of the gills found in most aquatic gastropods, the lining of the mantle cavity of terrestrial snails functions as a lung. • The class Bivalvia includes clams, oysters, mussels, and scallops. • Bivalves have shells divided into two halves. • The two parts are hinged at the mid-dorsal line, and powerful adductor muscles close the shell tightly to protect the animal. • Bivalves have no distinct head, and the radula has been lost. • Some bivalves have eyes and sensory tentacles along the outer edge of the mantle. • The mantle cavity of a bivalve contains gills that are used for feeding and gas exchange. • Most bivalves are suspension feeders, trapping fine particles in mucus that coats the gills. • Cilia convey the particles to the mouth. • Water flows into the mantle cavity via the incurrent siphon, passes over the gills, and exits via the excurrent siphon. • Most bivalves live rather sedentary lives, a characteristic suited to suspension feeding. • Sessile mussels secrete strong threads that tether them to rocks, docks, boats, and the shells of other animals. • Clams can pull themselves into the sand or mud, using the muscular foot as an anchor. • Scallops can swim in short bursts to avoid predators by flapping their shells and jetting water out their mantle cavity. • Cephalopods are active predators that use rapid movements to dart toward their prey, which they capture with several long tentacles. • Squids and octopuses use beak-like jaws to bite their prey and then inject poison to immobilize the victim. • A mantle covers the visceral mass, but the shell is reduced and internal in squids, missing in many octopuses, and exists externally only in chambered nautiluses. • Fast movements by a squid occur when it contracts its mantle cavity and fires a stream of water through the excurrent siphon. • By pointing the siphon in different directions, the squid can rapidly move in different directions. • The foot of a cephalopod has been modified into the muscular siphon and parts of the tentacles and head. • Cephalopods are the only molluscs with a closed circulatory system. • They also have well-developed sense organs and a complex brain. • The ancestors of octopuses and squid were probabl
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