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Read chapter 2 of the text. Phylum Chordata. The chordates are a group of particular interest to us as we belong to it, being members of the subphylum Vertebrata.
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Phylum Chordata • The chordates are a group of particular interest to us as we belong to it, being members of the subphylum Vertebrata. • The chordates include all of the vertebrates (fish, amphibians, reptiles, mammals and birds), but also two non-vertebrate subphyla: the Urochordata and the Cephalochordata.
Phylum Chordata • The chordates were in the 19th century considered to have been derived from protostome ancestors (the annelid, mollusc, arthropod group). • However, a better understanding of embryology shows that chordates are deuterostomes and the invertebrates they are most closely related to are the Echinodermata (sea stars, sand dollars, sea urchins) and the Hemichordata (acorn worms).
Protostomes and Deuterostomes • Within the eucoelomates there are two major evolutionary lineages that split early in the history of animals and follow quite different developmental pathways. These are the protostomes “mouth first” and deuterostomes “mouth second”.
Important differences in development between protostomes and deuterostomes • The differences in development that distinguish the protostomes and deuterostomes include: • Whether cleavage of cells in the early zygote is spiral or radial. • Whether or not, if the early blastomere is separated, each cell can develop into a normal larva or not. • Whether the blastopore ultimately forms the mouth or anus of the organism. • Whether or not the organism possesses a coelom and how that coelom is formed.
Protostomes and Deuterostomes • Protostomes include the annelids, mollusks, and arthropods. • Deuterostomes include the echinoderms and Chordates, which includes the vertebrates.
Characteristics of the Chordata • Chordates are: • bilaterally symmetrical • triploblastic • have a well developed coelom • have a complete digestive system
Germ layers of a triploblastic organism • Endoderm: innermost germ layer of an embryo. Forms the gut, liver, pancreas. • Ectoderm: Outer layer of cells in early embryo. Surrounds the blastocoel. Forms outer epithelium of body and nervous system. • Mesoderm: Third germ layer formed in gastrula between ectoderm and endoderm. The coelom forms in the mesoderm. Mesoderm gives rise to connective tissue, muscle, urogenital and vascular systems and peritoneum.
Coeloms • The coelom is a cavity entirely surrounded by mesoderm. • A coelom provides a tube-within-a-tube arrangement which has many advantages: • Allows visceral organs to grow independently of the body wall • fluid-filled coelom acts as a hydrostatic skeleton in some animals (e.g. earthworms). • In mammals the pericardial, peritoneal, and pleural cavities are formed from the coelom.
Coeloms • Triploblastic organisms (organisms with three germ layers including mesoderm fall into one of three different coelomic states: • Acoelomate: mesoderm fills the blastoceol, no cavity occurs in the mesoderm. Flatworms and nemerteans. • Pseudocoelomate: mesoderm lines only outer edge of blastocoel. No peritoneal lining develops. Nematodes and rotifers. • Eucoelomate: Have a true coelom derived from mesoderm and lined with peritoneum. Arthropods, annelids, mollusks, echinoderms, vertebrates.
Five distinctive characteristics of the chordates • Five distinctive characteristics separate the chordates from all other phyla: • Notochord • Single, dorsal, tubular nerve cord • Pharyngeal pouches or slits • Endostyle • Postanal tail • Not all of these characteristics are apparent in adult organisms and may appear only in the embryonic or larval stages.
Notochord • Notochord: the notochord is a flexible, rodlike structure. It extends the length of the body and is an anchor point for muscles. • The notochord bends without shortening so it permits the animal to undulate.
Notochord • In nonvertebrates and the jawless vertebrates the notochord is present throughout life. • However, in the jawed vertebrates it is replaced by the vertebral column; the remnants of the notochord being found in the intervertebral disks.
Single, dorsal, tubular nerve cord • In most invertebrates the nerve cord, if present, is ventral to the gut. • In chordates, in contrast, the nerve cord is dorsal to the gut and notochord. The nerve cord passes through the neural arches of the vertebrae, which protect it. • The nerve cord is enlarged in vertebrates into a brain, which is surrounded by a bony or cartilaginous cranium.
Pharyngeal pouches and slits • Pharyngeal slits occur in aquatic chordates and lead from the pharyngeal cavity to the outside. • The pharyngeal slits are used as a filter feeding device in protochordates (i.e., Urochordata (Tunicates)) and Cephalochordata (lancelets e.g. Amphioxus). • Water containing food is drawn in through the mouth by cilia and exits via the pharyngeal slits where the particles are trapped in mucus.
Pharyngeal pouches and slits • In vertebrates the pharyngeal arches have been modified into gills by the addition of a rich blood supply and thin gas permeable walls. • The contraction of muscles in the pharynx drive water through the gills.
Pharyngeal pouches and slits • In amniotes an opening may not form and rather than slits only grooves called pharyngeal pouches develop. • In tetrapods these pouches give rise during development to a variety of structures including the middle ear cavity, eustachian tube, and tonsils.
Endostyle or thyroid gland • The endostyle is found in protochordates and in lamprey larvae. It is located on the floor of the pharynx and secretes mucus, which is used to trap particles. • The endostyle works with the pharyngeal slits in filter feeding.
Endostyle or thyroid gland • Some cells in the endostyle secrete iodinated proteins and are homologous with iodinated-hormone secreting thyroid gland, which is found in adult lampreys and vertebrates.
Postanal tail • The postanal tail, some musculataure and the notochord enable larval tunicates and amphioxus to swim. • The postanal tail evolved to allow organisms to swim and its efficiency has been enhanced by the addition of fins. The postanal tail is present only in vestigial form in humans (the coccyx) although tails as a whole are widespread among vertebrates.
Classification of the Chordata • There are three subphyla in the Chordata: • Subphylum Urochordata: tunicates • Subphylum Cephalochordata: lancelets • Subphylum Vertebrata: fish, amphibians, reptiles, birds, mammals, etc.
Subphylum Urochordata • The Urochordata (“tunicates” named for the tough tunic that surrounds the adult) look like most unpromising candidates to be chordates and relatives of the vertebrates. • The largest group, the ascidians or sea squirts (Class Ascidiacea) as adults are marine, sessile, filter feeding organisms that live either solitarily or in colonies.
Ciona intestinalis (a solitary sea squirt)
Ascidians • Adult ascidians lack a notochord and there is only a single ganglion in place of the dorsal nerve cord. • Of the five characteristics of chordates adults possess only two: pharyngeal gill slits and an endostyle, both of which they use in filter feeding.
Ascidians • The adult sea squirt draws water in through an incurrent siphon and pushes it back out an excurrent one. • Food particles are filtered out in the pharyngeal slits with mucus from the endostyle used to trap particles.
Larval Ascidian • Even though the adult ascidian hardly resembles a chordate its larva does. • Larval ascidians are very small and tadpole-like and possess all five chordate characteristics previously outlined.
Larval Ascidian • The larval ascidian’s role is to disperse and to achieve this it is free swimming. However, it has only a short larval life (minutes to a couple of days) and does not feed during this time. • Instead it searches for a place to settle and then attaches and metamorphoses into an adult.
Ascidian metamorphosis • During metamorphosis the notochord disappears, the nerve cord is reduced to a single nerve ganglion and a couple of nerves.
Other Urochordate classes • Besides the ascidians there are two other classes of the Urochordata: the Larvacea and Thaliacea. • Both are small, transparent planktonic forms. Thaliaceans are cylindrical or spindle shaped whereas larvaceans are tadpolelike and resemble an ascidian larva.
Garstang’s hypothesis of chordate larval evolution • In the 1920’s it was proposed that the vertebrates were derived from an ancestral ascidian that retained its characteristics into adulthood (the process by which juvenile characteristics are retained into adulthood is referred to as paedomorphosis).
Garstang’s hypothesis of chordate larval evolution • Garstang’s hypothesis is supported by embryological evidence, but more recently molecular analyses have suggested that sessile ascidians are a derived form and that the free-living larvaceans are more likely to be the closest relatives of the chordates.
Subphylum Cephalochordata • The cephalochordates are the lancelets, which are small (3-7 cm long) laterally compressed fishlike animals that inhabit sandy sediments of coastal waters. They lack a distinct head and have no cranium. • They are commonly referred to as Amphioxus as this was the original genus name. There are 29 species, five of which occur in North American coastal waters.
Amphioxus • Amphioxus is a filter feeder. • Water enters the mouth and then is moved by beating cilia through the pharyngeal slits, where food is trapped in mucus. Cilia then move the food to the gut.
