Chapter 1 Phylum Protochordates.pptx
- 1. Chapter no 1 Lecture 1
- 2. The phylum Hemichordata includes the acorn worms and the pterobranchs. Members of both classes live in or on marine sediments. Characteristics of the phylum Hemichordata include: Marine, deuterostomate animals with a body divided into three regions: proboscis, collar, and trunk; coelom divided into three cavities (tripartite coelom) Ciliated pharyngeal slits Open circulatory system Complete digestive tract Dorsal, sometimes tubular, nerve cord Phylum Hemichordata
- 3. Members of the Class Enteropneusta are marine worms that usually range in size between 10 and 40 cm, although some can be as long as 2 m. The common name of the enteropneusts—acorn worms—is derived from the appearance of the proboscis, which is a short, conical projection at the worm’s anterior end. A ring like collar is posterior to the proboscis, and an elongate trunk is the third division of the body. Class Enteropneusta
- 4. A ciliated epidermis and gland cells cover acorn worms. The mouth is located ventrally between the proboscis and the collar. Varying numbers of pharyngeal slits, from a few to several hundred, are positioned laterally on the trunk. Pharyngeal slits are openings between the anterior region of the digestive tract, called the pharynx, and the outside of the body. A small diverticulum of the gut tract called the buccal diverticulum extends into the proboscis.
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- 6. Enteropneusts are dioecious. Two rows of gonads lie in the body wall in the anterior region of the trunk, and each gonad opens separately to the outside. Fertilization is external. Ciliated larvae, called tornaria, swim in the plankton for several days to a few weeks. The larvae settle to the substrate and gradually transform into the adult form. The similarity of the tornaria larva to echinoderm larvae provides one source of evidence of the close evolutionary ties between the hemichordates and echinoderms. Reproduction and Development
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- 8. Characteristics of the phylum Chordata include: Bilaterally symmetrical, deuterostomate animals A unique combination of five characteristics present at some stage in development: notochord, pharyngeal slits or pouches, dorsal tubular nerve cord, post anal tail, and an endostyle or thyroid gland Complete digestive tract Ventral, contractile blood vessel (heart Phylum Chordata
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- 10. The phylum is named after the notochord a supportive rod that extends most of the length of the animal dorsal to the body cavity and into the tail. It consists of a connective-tissue sheath that encloses cells, each of which contains a large, fluid-filled vacuole. At the same time, the notochord is flexible enough to allow lateral bending, as in the lateral undulations of a fish during swimming. In most adult vertebrates, cartilage or bone partly or entirely replaces the notochord. Notochord
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- 12. Pharyngeal slits are a series of openings in the pharyngeal region between the digestive tract and the outside of the body. The earliest chordates used the slits for filter feeding; some living chordates still use them for feeding. Other chordates have developed gills in the pharyngeal pouches for gas exchange. The pharyngeal slits of terrestrial vertebrates are mainly embryonic features and may be incomplete. Pharyngeal slits
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- 14. The tubular nerve cord and its associated structures are largely responsible for chordate success. The nerve cord runs along the longitudinal axis of the body, just dorsal to the notochord, and usually expands anteriorly as a brain. This central nervous system is associated with the development of complex systems for sensory perception, integration, and motor responses. Tubular Nerve Cord
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- 16. A fourth chordate characteristic is a postanal tail. (A postanal tail extends posteriorly beyond the anal opening.) Either the notochord or vertebral column supports the tail. The fifth characteristic unique to chordates is the presence of an endostyle or thyroid gland. An endostyle is present on the ventral aspect of the pharynx in tunicates, cephalochordates, and larval lampreys. It secretes mucus that helps trap food particles during filter feeding. In adult lampreys and other chordates, the endostyle is transformed into an endocrine structure, the thyroid gland. Postanal Tail and Thyroid Gland.
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- 19. Members of the subphylum Urochordata are the tunicates or sea squirts. The ascidians comprise the largest class of tunicates. They are sessile as adults and are either solitary or colonial. The appendicularians and thaliaceans are planktonic as adults. Sessile urochordates attach their saclike bodies to rocks, pilings, ship hulls, and other solid substrates. Subphylum Urochordata
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- 21. The unattached end of urochordates contains two siphons that permit seawater to circulate through the body. One siphon is the oral siphon, which is the inlet for water circulating through the body and is usually directly opposite the attached end of the ascidian. It also serves as the mouth opening. The second siphon, the atrial siphon, is the opening for excurrent water. Siphon
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- 23. The body wall of most tunicates is a connective- tissue-like covering, called the tunic, that appears gel-like but is often quite tough. Secreted by the epidermis, it is composed of proteins, various salts, and cellulose. Some mesodermally derived tissues, including blood vessels and blood cells, are incorporated into the tunic. Tunic
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- 25. Longitudinal and circular muscles below the body-wall epithelium help change the shape of the adult tunicate. The nervous system of tunicates is largely confined to the body wall. It forms a nerve plexus with a single ganglion located on the wall of the pharynx between the oral and atrial openings. Tunicates are sensitive to many kinds of mechanical and chemical stimuli, and receptors for these senses are distributed over the body wall, especially around the siphons. Maintenance Functions
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- 27. There are no complex sensory organs. The most obvious internal structures of the urochordates are a very large pharynx and a cavity, called the atrium, that surrounds the pharynx laterally and dorsally. Numerous pharyngeal slits called stigmas perforate the pharynx. Cilia associated with the stigmas cause water to circulate into the pharynx, through the stigmas, and into the surrounding atrium. Sensory Organs
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- 29. The digestive tract (gut) of adult tunicates continues from the pharynx and ends at the anus near the atrial siphon. The endostyle is a ventral ciliated groove that forms a mucous sheet. Digestive enzymes are secreted in the stomach, and most absorption occurs across the walls of the intestine. In addition to its role in feeding, the pharynx also functions in gas exchange. Gases are exchanged as water circulates through the tunicate. Digestive System
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- 31. The tunicate heart lies at the base of the pharynx. One vessel from the heart runs anteriorly under the endostyle, and another runs posteriorly to the digestive organs and gonads. Blood flow through the heart is not unidirectional. Peristaltic contractions of the heart may propel blood in one direction for a few beats; then the direction is reversed. Tunicate blood plasma is colorless and contains various kinds of amoeboid cells Circulatory System
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- 33. Ammonia diffuses into water that passes through the pharynx and is excreted. In addition, amoeboid cells of the circulatory system accumulate uric acid and sequester it in the intestinal loop. Pyloric glands on the outside of the intestine are also thought to have excretory functions Excretory System
- 34. Urochordates are monoecious. Gonads are located near the loop of the intestine, and genital ducts open near the atrial siphon. Gametes may be shed through the atrial siphon for external fertilization, or eggs may be retained in the atrium for fertilization and early development. Although self-fertilization occurs in some species, cross-fertilization is the rule. Reproduction and Development
- 35. Development results in the formation of a tadpole like larva with all five chordate characteristics. Metamorphosis begins after a brief free-swimming larval existence, during which the larva does not feed. The larva settles to a firm substrate and attaches by adhesive papillae located below the mouth. Reproduction and Development
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- 38. Members of the subphylum are called lancelets. Lancelets clearly demonstrate the five chordate characteristics, and for that reason they are often studied in introductory zoology courses. The cephalochordates consist of two genera, Branchiostoma (amphioxus) and Asymmetron, and about 45 species. Cephalochordates are small (up to 5 cm long), tadpole like animals. They are elongate, laterally flattened, and nearly transparent. Subphylum Cephalochordata
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- 40. In spite of their streamlined shape, cephalochordates are relatively weak swimmers and spend most of their time in a filter feeding. The notochord of cephalochordates extends from the tail to the head, giving them their name. Segmentally arranged muscle cells on either side of the notochord cause undulations and a median dorsal fin and a caudal fin also aid in swimming. Cephalochordates
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- 42. Cephalochordates are filter feeders. During feeding, they are partially or mostly buried in sandy substrates with their mouths pointed upward. Cilia on the lateral surfaces of gill bars sweep water into the mouth. Water passes from the pharynx, through pharyngeal slits to the atrium, and out of the body through the atriopore. Maintenance Functions
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- 44. Food is initially sorted at the cirri. Larger materials catch on cilia of the cirri. Smaller, edible particles are pulled into the mouth with water and are collected by cilia on the gill bars and in mucus secreted by the endostyle. Digestion is both extracellular and intracellular. A diverticulum off the gut, called the midgut cecum, extends anteriorly. It ends blindly along the right side of the pharynx and secretes digestive enzymes. An anus is on the left side of the ventral fin. Digestive System
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- 46. Cephalochordates do not possess a true heart. Blood contains amoeboid cells and bathes tissues in open spaces. Excretory tubules are modified coelomic cells closely associated with blood vessels. The coelom of cephalochordates is reduced, compared to that of most other chordates. It is restricted to canals near the gill bars, the endostyle, and the gonads. Other Systems
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- 48. Cephalochordates are dioecious. Gonads bulge into the atrium from the lateral body wall. Gametes are shed into the atrium and leave the body through the atriopore. External fertilization leads to a bilaterally symmetrical larva. Larvae are free swimming, but they eventually settle to the substrate before metamorphosing into adults. Reproduction and Development
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