ZOO 2040 Biology of Animals Topic 12 Lophophorates, Chaetognaths, Echinoderms, and Hemichordates

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1 Deuterostomes are a group of several higher phyla: the Lophophorates (Phoronida, Ectoprocta, and Brachiopoda), Chaetognatha, Echinodermata, Hemichordata, and Chordata (which includes vertebrates) Lophophorates The phyla Phoronida, Ectoprocta and Brachiopoda are coelomate, lack a distinct head and possess a lophophore: A unique arrangement of ciliated tentacles is on a ridge that surrounds the mouth and is used to collect food. The lophophore is extended for feeding and withdrawn for protection. Within the tentacles is an extension of the coelom that also serves for gas exchange. All three phyla have a U-shaped alimentary canal: The anus is near the mouth but outside of the lophophore. The coelom is divided into three regions: the protocoel, mesocoel and metacoel: The mesocoel extends into the hollow tentacles of the lophophore. The protocoel, when present, forms a cavity in a flap over the mouth, the epistome. The body section containing the mesocoel is called the mesosome; a metasome contains the metacoel. Larvae of all three phyla are free-swimming and trochophorelike, but are sessile as adults. Phylum Entoprocta ( within anus ) Are PROTOSTOMES, but have a lophohore. Also! known as bryozoans About 150 spp. Tiny (most! microscopic, all <10 mm) and marine except for! Urnatella, which is freshwater. Widely distributed! in coastal areas. Stalked and sessile. Often grow on! shells and algae. Some are commensals of! polychaetes. Some are monoecious and form extensive colonies! asexually. Others are dioecious. Fertilized eggs develop in! a brood pouch between gonopore and anus. Modified! spiral cleavage leads to a trochophore-like larvae similar to annelids and molluscs. Larvae free-swimming before growing stalks and attaching. Resemble colonial hydrozoans, but have 4-36 non-retracting tentacles that are ciliated on their lateral and inner surfaces and can roll inward. Long lateral cilia maintain an incoming water current, short cilia on insides of tentacles direct microscopic animals and debris into the mouth. Body (calyx) is cup-shaped, attached by adhesive glands! on stalk (tentacles and stalk are continuations of body! wall (which has a cuticle, cellular epidermis, and! longitudinal muscles). Gut is U-shaped, with mouth and anus opening inside the ring of tentacles. No circulatory/respiratory organs. One pair of protonephridia! are embedded in gelatinous parenchyma (which fills their! pseudocoel ). Phylum Ectoprocta Have also been called bryozoans ( moss animals ) Excluded as pseudocoelomates because anus is located within the tentacular crown Zoologists that continue to use the term bryozoa exclude entoprocts from the group. About 4000 spp., with a fossil record extending back to the Ordovician Diverse and abundant group of colony builders; most <0.5 mm long, Grow on firm underwater surfaces, both freshwater and marine (but primarily in shallow water) Colonies may resemble! limey encrustations,! fuzzy growths or! seaweed-like colonies Many resemble hydroids,! but have an anus Colonies vary from! gelatinous masses a few! cm across to encrusted! colonies 1 m wide Most colonies have uniform! feeding individuals; a few! have specialized forms to! dispose of debris Ectoprocta (cont.) Each member of a colony lives! in a chamber called a zoecium! secreted by the epidermis.! The shape of the! zoecium may be! boxlike, vase-like, or! tubular: Each individual zooid has! a feeding polypide and a! case-forming cystid. The cystid, together with! its secreted exoskeleton,! forms the body wall of the animal. The zoecium may be gelatinous, chitinous, or hardened with calcium. The polypide pops up to feed and then quickly withdraws into a little chamber under a trapdoor. Sets of muscles can extend or withdraw the lophophore crown. Lophophore ridge is circular in marine spp.; U-shaped in freshwater spp. Particles are drawn in by both the cilia and pumping action of the pharynx. Ectoprocta (cont.) No respiratory, vascular, or excretory organs: Gaseous exchange through the body surface is sufficient; coelomocytes engulf waste materials. Digestion appears to be extracellular for proteins and starches, and intracellular for fats. Most ectoprocts are hermaphroditic. Reproduce asexually by budding to form colonies: Some shed eggs into seawater; most brood their eggs either in the coelom or in a modified zoecium. Cleavage is radial and apparently mosaic Larvae of nonbrooding species are free-swimming for a few months; larvae of brooding species settle. They attach to substrate by mucopolysaccharide and protein secretions from an adhesive sac. Freshwater ectoprocts also bud to produce statoblasts (hard, resistant capsules of cells) that overwinter and give rise to new polypides and new colonies the following spring. Bugula species can be invasive and a pest (causing fouling when colonies are broken up), but their zoecia are being investigated for anti-cancer properties. Page 1

2 Phylum Brachiopoda ( arm foot ) Lamp shells : over 30,000 fossil spp. are known from! Paleozoic and Mesozoic seas (to about 550 Mya). The 350! living spp. are little changed from fossil forms (Lingula is! the oldest of all modern genera). Modern brachiopods range from 5 to 80 mm in length; fossil! forms reached 30 cm. Until 100 years ago, were considered to be mollusks: they! have two calcareous shells and resemble bivalves: Valves are dorsal and ventral, unlike the right and left valves of Bivalvia. Most have the ventral valve larger than the dorsal valve. (Fig. 22-7). Most attach to the substrate directly or by a fleshy stalk or pedicel. In some species, the pedicel valve resembles the ancient Greco-! Roman oil lamps, hence their name. Class Inarticulata (50 spp in 2 orders): shells are held! together by muscles alone: Class Articulata (300 spp in 2 orders): have a connecting! hinge with an interlocking tooth-and-socket arrangement. Phylum Brachiopoda (cont.) The body occupies the posterior of the cavity between the valves The posterior metacoel bears the viscera The anterior cavity holds the large horseshoe-shaped lophophore with ciliated tentacles Organic detritus and some algae are the main food carried along the lophophore to the mouth. One or two pairs of nephridia open into the coelom and empty into the mantle cavity Open circulatory system with a contractile heart The lophophore and mantle are the main sites of gas exchange The nerve ring connects the small dorsal and larger ventral ganglion Sexes are separate; paired gonads discharge gametes through the nephridia. Most fertilization is external but a few species brood their eggs and young. Cleavage is radial and coelom formation is enterocoelic. There is no metamorphosis; the adult phase begins when the larva settles. Phylum Phoronida About 10 spp. of small, wormlike animals. Epibenthic in shallow, coastal areas of! temperate seas. Few millimeters to 30 cm in length. Secrete a leathery or chitinous tube,! which they never leave: The lophophore is thrust out to feed; it is! then withdrawn into the tube. The mouth is between the two ridges of the! lophophore that forms a horseshoe shape. The horns of the ridges often coil into twin! spirals. Cilia on the tentacles direct a water current! toward the groove between the ridges and! to the mouth. Cilia in the stomach area of the gut aid in food! movement. The anus lies dorsal to the mouth, outside the lophophore, and has a nephridiopore on each side: Water leaving the lophophore washes past anus and nephridiopores. Phylum Phoronida The body wall has a cuticle and! epidermis; has both longitudinal and! circular muscles. The protocoel connects at the side to the! mesocoel; a septum separates the! metacoel from the mesocoel. They have a closed system of contractile! blood vessels but no heart: Have nucleated red blood cells with hemoglobin Two metanephridia for excretion. A nerve ring communicates to the tentacles! and body wall; one giant motor fiber lies! in the epidermis. Most are monoecious but some are! dioecious; one reproduces asexually: Cleavage is radial. Coelom formation is enterocoelus but the! blastopore becomes the mouth. A free-swimming larva called an actinotroch metamorphoses into a sessile adult that secretes a tube. Phylogeny and Adaptive Radiation Phylogeny Some consider all three phyla to be a monophyletic clade, and that they are deuterostomes. Sequence analysis of rrna suggests they are protostomes. They appear close to Annelida and Mollusca and the lophophore is a unique synapomorphy (shared evolutionarily-derived structure). The three part coelom, and the homology of coelomic sections with other phyla are also questioned. The trochophore-type larvae do not closely resemble those of annelids and molluscs. Adaptive Radiation Lophophorate morphological diversity has been constrained by the limitations of filter-feeding. Brachiopod variations are constrained by life within a shell. The tubes of phoronids vary by habitat. Ectoprocts appear adapted to life at a small scale, as shown by zooids and colony formations. Phylum Chaetognatha Arrow worms : 120 spp. Small ( cm) and! torpedo-shaped. All are marine; specialized for planktonic life: Hugely abundant (outnumbered only by copepods in! marine zooplankton). Spend most of their time drifting, but their caudal fin allows! for short bouts of rapid swimming. Horizontal fins assist in! steering, orientation and flotation, but not swimming. Spadella (the only benthic genus with 20% of the species)! swims to surface at night. Most are nearly transparent, an advantage as planktonic! feeders. Some deep-water species are orange, and at least one! species has bioluminescent organs along each side. Morphology and embryology places this group with the! deuterostomes (embryonic cleavage is radial, complete! and equal): Their embryogenesis differs from other deuterostomes, however. in that the coelom is formed by a backward extension from the archenteron. They have some developmental similarities to nematodes, and molecular evidence also suggests that they are protostomes. A vestibule under the head leads to the mouth; curved spines on each side help seize prey: Head has a hood that can be drawn over the eyes and spiny teeth. As the hood pulls back, the spines spread apart to snatch food and then retract. Some species produce tetrodotoxin, perhaps to subdue prey. Page 2

3 Chaetognatha (cont.) The body is unsegmented. A thin cuticle covers! the body and there is a multi-layered! epidermis on the sides. They have a complete digestive system and a! well-developed coelom, but no true peritoneum. The nervous system has a nerve ring with large! dorsal and ventral nerve cords (there are also! lateral ganglia). Sense organs include a pair of compound eyes! (with 5 ocelli each), and sensory bristles. No vascular, respiratory, or excretory systems. Solid wastes pass out of the anus. Other wastes pass through the skin. Hermaphroditic with both cross- and self-fertilization: Eggs of Sagitta are planktonic and have a coat of jelly. Other arrow worm eggs may be attached to the body and carried by the mother. Juveniles develop directly without metamorphosis. Cleavage is radial, complete and equal. Fossilize poorly, but are thought to have originated during the Cambrian: Grasping spines are found occasionally as fossils from the late Paleozoic onward. Previously, the oldest fossils were from Mazon Creek, IL (300 Mya). Complete body fossils that have not been formally described are also reported from the Burgess Shale and from Lower Cambrian shales in China (a fossil species dating from 520 Mya in China was described in Science in 2002). Possibly related to some of the animals grouped with the! conodonts. It is now thought that these protoconodonts,! known only from their teeth, might be chaetognaths! rather than ancestors of vertebrates. sea urchin Phylum Echinodermata ( spiny skin ) Ancient phylum, extending back to the! Cambrian. About 7,000 spp. In addition to! the six living classes, there are several! extinct classes. brittlestars They have a false radial (pentamerous)! symmetry: Larvae are bilateral (and adults really are also). One theory is that they evolved a radial body! as an adaptation to a sessile existence. The! body plan is derived from crinoid-like ancestors that evolved freemoving descendants later. All have an endoskeleton consisting of dermal calcareous ossicles with spines covered by dermis (or spicules in dermis). They have a unique water-vascular system, derived from a coelomic compartment, that extends out from the body surfaces as many tube feet. May have pedicellariae (tiny pincers used to clean the outside body) and dermal branchiae (soft, gill-like skin structures). Due to their spiny structure, echinoderms are not often preyed upon: A few fishes and sea otters are adapted to feed on sea urchins. Humans sometimes eat sea urchin eggs/gonads ( roe ) and the body walls of certain sea cucumbers ( trepang ). No parasitic forms are known. A few are commensals, and some fishes and invertebrates live as commensals among the spines or even within the bodies of echinoderms. Asteroids or sea stars are mostly predators: Feed on molluscs, crustaceans, and other invertebrates; they may damage oyster beds. Sea stars of the genus Pisaster, however, help maintain biodiversity of attached marine life in rocky intertidal habitats in the North Pacific. Ophiuroids (brittle stars) are active scavengers, browsers or commensals: Basket stars damage coral reefs in Australia. Holothuroids (sea cucumbers) are mostly suspension or deposit feeders. Echinoids (sea urchins) are found on hard bottoms, while sand dollars prefer sand substrate; they feed on detritus or algae: Sea urchins that overgraze kelp cause depletion of other Pacific coast species. Commonly used in embryology, as their development is easy to observe (display artificial parthenogenesis: develop without fertilization if treated with hypertonic seawater or subjected to other stimuli). Crinoids are sessile and flower-like as young and may detach as adults; they are suspension feeders. Echinoderm Distribution Because of their water vascular system, echinoderms lack the ability to osmoregulate, and are restricted to marine environments. Range from the intertidal, down the continental shelf, and across the abyssal plains of the deep ocean bottoms (where many species live in huge numbers). slime star weird! sea! cucumber sea pig Class Asteroidea ( star-like ) About 1800 spp., sea stars are! common along shorelines and may! aggregate on rocks. Some sea stars live on muddy or! sandy bottoms, or among coral reefs. Range from 1 cm to about 1 m! across and may be brightly colored. Asterias spp. are common on the! east coast of the U.S.; Pisaster spp.! are common on the west coast. Sea stars consume a wide range of! food; some eat sea urchins and! regurgitate undigested parts. Some feed on molluscs; they pull! steadily until they can insert their! stomach through the crack. Some sea stars feed on small particles that are carried up ambulacral grooves in their arms to the mouth. Asteroid Form and Function Body plan is a central disc with tapering! arms extending outward: Body is flattened and flexible, with a pigmented! and ciliated epidermis. The mouth is on the underside or oral side. Ambulacrum runs from the mouth to the tip of! each arm. Usually there are five arms but there may be more. Ambulacral groove is bordered by rows! of tube feet: A large radial nerve is in the center of each groove. Under the nerve is an extension of the coelom and! the radial canal of the water-vascular system. In all other classes except crinoids, ossicles or other! dermal tissue covers these structures. The aboral side is spiny; at the base of the spines are! groups of pincer-like pedicellariae: On the aboral side is the anus, a circular madreporite (a sieve leading to the water-vascular system) and pedicellariae that clean the body surface. Papulae (dermal branchiae or skin gills) are soft projections lined with peritoneum and serve in gas exchange. Page 3

4 Asteroid Form! and Function (cont.) Endoskeleton Under the epidermis is the! mesodermal endoskeleton of! small calcareous plates or ossicles. Ossicles are penetrated by a! meshwork of spaces filled! with fibers and dermal cells! (steroem). Muscles in the body wall move the rays! and partially close the ambulacral grooves. There is a spacious body coelom filled with fluid in one coelomic compartment: The fluid contains amebocytes (coelomocytes). Ciliated peritoneal lining of the coelom circulates the fluid around the cavity and into papulae. Respiratory gases and nitrogenous waste ammonia diffuse across the papulae and tube feet. Some wastes are picked up by coelomocytes, which migrate to the tips of papulae to be pinched and their contents released. Water-Vascular System Unique to echinoderms, this system is! another coelomic compartment: It consists of a system of canals,! tube feet and dermal ossicles. Functions in locomotion, food-! gathering, respiration and excretion. Opens to the outside at the! madreporite on the aboral side: Madreporite leads to the stone canal,! which joins the ring canal that! encircles the mouth. Radial canals diverge from the! ring canal and extend into each ray: Four or five pairs of Tiedemann s bodies attach to the ring canal and may produce coelomocytes. Polian vesicles may also be attached; they serve for fluid storage. Small lateral canals, each with a one-way valve, connect the radial canal to the tube feet: The inner end of each tube foot or podium is an ampulla that lies within the body coelom. The outer end of each tube foot bears a sucker. Water-Vascular System (cont.) The system operates hydraulically;! valves in lateral canals! prevent backflow: Muscles in the ampullae! contract, forcing fluid into! and extending the podium. Contraction of longitudinal muscles! in the tube foot retracts it, forcing! fluid back into the ampullae. Small muscles in the end of the! tube foot raise the middle of! the disc-like end, creating suction. The sea star can move while firmly! adhered to the substrate. This system can exert a strong pulling force (1,300 g) on the shells of bivalves, fatiguing their adductor muscles and causing them to open. Tube feet are innervated by a central nervous system; they move in one direction but not in unison: Cutting a radial nerve ends coordination in one arm; cutting a circumoral nerve ring stops all movement. Digestive System The mouth on the oral side! leads through a short! esophagus to a large! central stomach. Some feed on particulates, and! small prey can be ingested! whole, but to consume! larger prey, the lower! cardiac part of the! stomach can be everted! through the mouth during feeding. After feeding, the stomach is retracted back inward by contracting the stomach muscles and relaxing the body wall muscles. The upper stomach is smaller and is connected by ducts to a pair of pyloric ceca in each arm. The anus is inconspicuous and empties in the center at the top; some asteroids lack an intestine and anus. Hemal System A system of tissue strands enclosing! unlined sinuses; it is itself enclosed! in perihemal channels. Function is not known. It might be! useful in distributing digested! products but this is unproven. Nervous System The oral system of a nerve ring and! radial nerves coordinates movements! of the tube feet. A deep hyponeural system aboral to! the oral system forms a ring around! the anus and extends into the roof of each ray. The epidermal nerve plexus coordinates responses of the dermal branchiae to tactile stimulation. Tactile organs are scattered over the surface and there is an ocellus at the tip of each arm. They react to touch, temperature, chemicals and light intensity; they are mainly active at night. Most have separate sexes; a pair of gonads is in each interradial space: Fertilization is external; in early summer, eggs and sperm are shed into the water Can regenerate lost parts; casting off injured arms and regenerating new ones. An arm can regenerate a new sea star if at least 20% of central disc remains. In most cases, embryonating eggs are dispersed in the water and hatch to free-swimming larvae: Embryogenesis shows a typical primitive deuterostome pattern. Coelomic compartments, called somatocoels, arise from legs of the posterior blastocoel. The left hydrocoel becomes the water-vascular system; the left axocoel becomes the stone canal and perihemal channels. Free-swimming larva has cilia arranged in bands and is called a bipinnaria: Ciliated tracts become larval! arms. When the larva grows three! adhesive arms and a sucker! at the anterior, it is called a! brachiolaria. It then attaches to the! substrate and metamorphizes! into a radial juvenile. As arms and tube feet appear,! juvenile detaches from its stalk! and becomes motile. Page 4

5 ZOO 2040 Biology of Animals Topic 12 Class Ophiuroidea ( snake-like ) Ophiuroidea (cont.) Brittle stars, serpent stars, and basket stars: largest! group in terms of number of species (2,100) and! probably also in terms of abundance. Secretive and live on hard or sandy bottoms where! little light penetrates, often under rocks or in kelp! holdfasts. They come out at night to browse on food as deposit! feeders or suspension feed (there can be several! thousand per m2 in some areas). Many live in associations with other species, especially! sponges and sessile cnidarians Arms are slender and distinct from the central disc: Ten invaginations called bursae! open to the oral surface by genital slits at the bases of the arms Gonads on the wall of each bursa discharge ripe sex cells into the water for external fertilization Each jointed arm has a column of articulated ossicles called! vertebrae. Arms are moved in pairs for locomotion. They lack pedicellariae or papulae and the ambulacral! groove is closed and coated with ossicles. Tube feet lack suckers and ampullae; protrusion is generated by proximal muscles. Visceral organs are all in the central disc; the arms are too slender to contain them. Echinoidea (cont.) Sea urchins lack arms, but their tests show! their pentamerous symmetry. Occur from intertidal to deep oceans. The up-folding of the arms brings the! ambulacral areas up to the anal region: A ciliated siphon connects the esophagus to the intestine; food can be concentrated in the intestine. Hemal and nervous systems resemble those in asteroids. In irregular urchins, respiratory podia are arranged in fields called petaloids on the aboral surface. Sexes are separate; eggs and sperm are shed into the sea: Inside the test is Aristotle s lantern, a complex! set of chewing structures: Sea urchins eat algae; sand dollars filter particles! through their spines. Some brood young in depressions between the spines. Echinopluteus larvae of nonbrooding echinoids live a planktonic existence. Class Holothuroidea Holothuroidea (cont.) As their name suggests, these animals resemble! cucumbers. They are greatly elongated in the! oral- aboral axis. About 1,200 spp. Abundant on shallow to deep ocean bottoms.! Some species crawl on the ocean bottom, others! are found under rocks or burrow. Body wall is leathery with tiny ossicles in it;! some have dermal armor. Sea cucumbers cast out part of their viscera when! irritated; they must regenerate these tissues. The organs of Cuvier are expelled in the direction of an enemy; they are sticky and have toxins. One small fish, Carapus, uses the cloaca and respiratory tree of a sea cucumber for shelter: Use both the ventral tube feet and! muscular body waves to move: In some, locomotor tube feet are distributed! to all five ambulacral areas. In most, only the side that faces the substratum! (the sole) has (three) ambulacra with tube feet! (producing a secondary bilateral symmetry) tube feet (oral tentacles) around! the mouth Some trap particles on the mucus of their tentacles! and suck off the food in their pharynx; others graze the sea bottom with their tentacles. All tube feet, except oral tentacles, are absent in burrowing forms. Coelomic cavity has many coelomocytes Digestive system opens into a cloaca; a respiratory tree also empties into the cloaca. A madreporite lies free in the coelom; the hemal system is more developed than in other echinoderms. John Steinbeck and his friend Edward Ricketts (the marine biologist who was the model for Doc in Steinbeck s novels Cannery Row and Sweet Thursday, discovered this species on a trip to Baja California, and planned to name it Walterwinchella, after the controversial newspaper and radio journalist. Five converging teeth and! sometimes branched gills! encircle the peristome: Anus, genital pores, and! madreporite are aboral in the periproct region. Sand dollars and heart urchins have shifted the anus to the posterior. Echinoid tests have 10 double rows of plates with! movable, stiff spines: The tube feet extend along the five ambulacral rows; regular! urchins move by tube feet. The spines articulate on ball-and-socket joints moved! by small muscles at the bases; irregular urchins move! using their spines. Three-jawed pedicellaria on! long stalks are most common: Some species have pedicellariae! with poison glands that secrete! a toxin that paralyzes small prey. Most sea urchins have a hemispherical shape! with radial symmetry and long, sharp, serrated spines. Sand dollars and heart urchins ( irregular echinoids)! have become bilateral with short spines. Regeneration and autotomy are more pronounced than in sea stars Very fragile (often 50% or more of a population is in the process of regenerating one or more arms) Skin is leathery and surface cilia are mostly lacking. The larva is an ophiopluteus; has ciliated bands that extend onto delicate and beautiful larval arms In contrast to sea stars, they lack any attached phases during metamorphosis Class Echinoidea Madreporite is on oral surface Five movable plates act as jaws! and surround the mouth; there! is no anus Saclike stomach; no intestine! Water-vascular, nervous, and! hemal systems resemble those! of asteroids Sexes are usually separate, but a! few are hermaphroditic Respiratory tree also serves for excretion; gas exchange also occurs through the skin and tube feet. Sexes are separate; some are hermaphroditic. Single gonad considered a primitive character. Fertilization is external; free-swimming auricularia larva. A few brood their young inside the body or on the body surface. Fishing for sea cucumbers (trepang) by trawls is a major cause of reef destruction. Page 5

6 Class Crinoidea Sea lilies and feather stars Unique in that they are attached for most of their life Sea lilies have a flower-shaped body at the tip of a stalk Feather stars have long, many-branched arms; adults are free-! moving but may be sessile. Many crinoids are deep-water species; feather stars are! found in more shallow water. The body disc or calyx is covered with a leathery skin! or tegmen of calcareous plates. The five arms branch to form more arms, each with lateral! pinnules as in a feather. The calyx and arms form a crown. Sessile forms have a stalk formed of plates; it appears to be! jointed and may bear cirri. No madreporite, spines, or pedicellariae. Upper surface has a mouth that opens into an! esophagus and intestine; it then exits the anus. Tube feet and mucous nets allow it to feed on small! organisms in the ambulacral grooves. Have a water-vascular system, oral ring, and a radial nerve to each arm. Crinoidea (cont.) Reproduction Sexes are separate; gonads are merely masses of cells in cavities of the arms and pinnules: Gametes escape through ruptures in the pinnule wall. Some brood their eggs. Doliolaria larvae are free-swimming before they! become attached and metamorphose. Most living crinoids are cm long;! some fossil species had stalks 20 m long. Paleozoic crinoids were especially! diverse and abundant in the! area around what is today is! Crawfordsville, Indiana. They are also common in! Illinois limestone deposits. Class Concentricycloidea Sea daisies Small, disc-shaped animals discovered in deep water off New Zealand and described in Only two species are known: One species has a shallow, sac-like stomach. The other species has no digestive tract; a velum covers the oral surface and absorbs nutrients. They are pentaradial but have no arms. Tube feet are located around the periphery of the disc rather than in ambulacral areas. The water-vascular system has an outer ring and a hydropore homologous to the madreporite that connects the inner ring canal to the aboral surface. Phylogeny and Adaptive Radiation The fossil record is extensive but there are still many theories about echinoderm evolution. From the larvae, we know the ancestor was bilateral and the coelom had three pairs of spaces. One theory states sessile groups derived independently from free-moving adults with radial symmetry: Traditional views consider the first echinoderms sessile and radial, giving rise to free-swimming forms. Early forms may have had endoskeletal plates with stereom structure and external ciliary grooves: Carpoids may be an extinct variation, or a separate subphylum. Echinoids and holothuroids are related; the relationship of ophiuroids and asteroids is controversial. If their ancestors had a brain and sense organs, these were lost in adoption of radial symmetry. The basic body plan has limited their evolutionary opportunities to become parasites. Phylum Hemichordata Benthic in shallow marine waters. Most are worm-like and sedentary or sessile: some are colonial and live in secreted tubes. Occur nearly worldwide but are secretive, delicate and difficult to collect. Hemichordates are similar to other deuterostomes (echinoderms and chordates: Tricoelomate (coelom forms! three compartments). Enterocoelic coelom formation Radial cleavage. Ciliated tornaria larvae are nearly! identical to echinoderm bipinnaria! larvae. Like the chordates, have a muscular pharynx! with gill slits: Mainly ciliary-mucus feeders: the primary function of the branchial mechanism of the pharynx is food gathering. Also like the chordates, some have a dorsal hollow nerve cord. Their stomochord however, is not homologous to the chordate notochord. Page 6

7 Class Enteropneusta ( one-half string ) Acorn worms : about 70 spp. Sluggish, wormlike animals, ranging from! 20 mm to 2.5 m long. The mucus-covered body is divided into a! tongue-like proboscis, short collar and long! trunk: The proboscis is the most active part of the animal; it! examines and collects food. Cilia carry particles to the groove at the edge of the! collar and then to the mouth on the ventral side. Large particles are rejected by covering the mouth! with the edge of the collar. Live in burrows or under stones in intertidal zones. Burrow dwellers also use the proboscis to excavate! the burrow: They build a U-shaped burrow with two openings 1-30 cm apart. The proboscis is thrust out the front opening. Defecation occurs at the back opening, leaving piles of feces. Enteropneusta! (cont.) Feeding and digestion A small coelomic sac or! buccal diverticulum is at! the posterior end of the! proboscis. A slender canal! connects the protocoel! with a proboscis pore opening to the outside. Water taken in or expelled from cavities moves the animal forward. Gill pores on each side open into gill chambers that connect to gill slits on the side of the pharynx: No gills in the gill slits; some respiratory exchange occurs on vascular branchial epithelium. Ciliary currents keep a fresh supply of water circulating through the pharynx. Ciliary-mucus feeders: the branchial mechanism of the pharynx is food gathering: Food caught in the pharyngeal mucus is moved by cilia into the ventral part of the pharynx and esophagus and eventually passes into the intestine for digestion and absorption. Enteropneusta! (cont.) A middorsal vessel carries! colorless blood forward! above the gut: In the collar, the vessel! expands into a sinus! and heart vesicle. Blood enters a network! of blood sinuses called! the glomerulus, which is assumed to have an excretory function. Blood travels posteriorly through a ventral vessel below the gut and through many sinuses. Nervous system is a subepithelial plexus of nerve cells and fibers attached to epithelium: This net thickens to form dorsal and ventral nerve cords joined by a nerve ring. The collar cord is hollow in some cases and contains giant nerve cells; it resembles the nerve plexus of cnidarians and echinoderms. Sexes are separate; external fertilization: Two dorsolateral rows of gonads along each side of the anterior trunk. Tornaria larvae. Some have direct development without any tornaria stage. Class Pterobranchia Discovered on the Challenger! Expedition ( ). Resemble enteropneusts, but have! structures associated with sedentary! lifestyle, and have no tubular nerve! cord in the collar. Small, ranging from 1-7 mm long, and may! have a longer stalk. Many individuals may! live together in interconnected tubes: Each zooid is independent within its tube; they extend their crown of tentacles! from the apertures of these tubes. They are attached to the tube wall and! can retract the tentacles immediately. U-shaped alimentary canal, with anus near the mouth. At base of proboscis are 5-9 pairs of arms with! tentacles extending the coelomic compartments. Ciliated grooves on the arms collect food. Some are dioecious; others are monoecious: In Cephalodiscus, new individuals are asexually! budded from a creeping basal stolon. No gill clefts or glomeruli are present. Class Graptolitha ( map rocks ) Colonial, often planktonic marine animals. Most are extinct, but one living species has (may have?) been found. Very abundant as fossils about Mya Important index fossils of Ordovician and Silurian rocks (once used to date! rock strata). Related (and probably! gave rise) to the! pterobranchs (or was it! the other way round?). Phylogeny and Adaptive Radiation Aside from shared deuterostome features, chaetognaths are difficult to place: Sequence analysis of rrna places chaetognaths among protostomes. Others suggest chaetognaths originated independently from an early coelomic lineage. Hemichordates share pharyngeal gill slits (for filter feeding and respiring) with the chordates: The short, dorsal nerve cord in the collar may be homologous to the chordate nerve cord. Some investigations show that the cord is not hollow. A buccal diverticulum in the hemichordate buccal cavity is not homologous to the chordate notochord. Embryos of hemichordates are very similar to those of echinoderms: The tornaria larva is nearly identical to a bipinnaria larva. Placement of hemichordates with other lophophorates is not supported by molecular evidence. A sessile life in ocean bottoms has allowed little adaptive divergence in Pterobranchs. Having lost tentaculated arms and using a proboscis, enteropneusts diverged only a little more. Page 7