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. 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.

Amphioxus

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.

Amphioxus

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)

Synoicum pulmonaria  a colonial 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.

15.4

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.

Young larval ascidian

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.

15.5

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.

Amphioxus

Amphioxus Amphioxus is interesting because it displays the basic chordate characteristics in a simple and obvious form because of its transparency. Amphioxus is considered to be the closest living relative of the vertebrates because it shares several characteristics with vertebrates that Urochordates do not possess.

Amphioxus characteristics shared with vertebrates Characteristics Amphioxus shares with vertebrates include: Segmented myomeres (blocks of striated muscle separated by connective tissue) Dorsal and ventral aortas Branchial (gill) arches (blood vessels running over the gills).

Amphioxus characteristics not shared with vertebrates Amphioxus however lacks several characteristics that biologists think the ancestor of vertebrates possessed. These include: Tripartite brain (with forebrain, midbrain and hindbrain) protected by a cranium (skull) Chambered heart Closed circulatory system Muscular gut and pharynx (food moved through gut by ciliary action not peristalsis) List continues on next slide

Amphioxus characteristics not shared with vertebrates Various special sensory organs (eyes, chemical and pressure receptors, nose, inner ear) Neural crest (ectodermal cells that are found on the embryonic neural tube and are engaged in the formation of the cranium, tooth dentine, some endocrine glands and Schwann cells, which provide myelin insulation to nerve cells).

Significance of differences between amphioxus and vertebrates The differences between non-vertebrate chordates such as Amphioxus and early (and modern) vertebrates are a result of the increased size and activity of vertebrates.

Significance of differences between amphioxus and vertebrates Because vertebrates are large they cannot depend on slow processes such as diffusion and ciliary action to support them. As a result, specialized organ systems are needed to carry out physiological tasks at a much faster rate.

Significance of differences between amphioxus and vertebrates Vertebrates are also much more active than nonvertebrate chordates and are predators or at least grazers (which is “predation” on plants). Activity requires a suite of traits to support it. sensory systems so you can seek things out; complex nervous system to coordinate activity; more efficient circulatory, respiratory and digestive systems to fuel the activity; muscles and skeleton to facilitate movement. The transition from nonvertebrate chordate to vertebrate thus is closely related to the adoption of a more active lifestyle.

Haikouella An early stage in the transition to vertebrates is marked by the Cambrian era (520mya) fossil Haikouella Haikouella is likely similar to the common ancestor of the vertebrates and it possesses a muscular pharynx, which implies it pumped water across its gills, which implies in turn a more active lifestyle, although it also possesses a filter-feeding apparatus.

Once physiological systems began to develop that enabled vertebrates to be more active and sense their environments we would expect ecological competition and selection to have rapidly driven the evolution of vertebrate diversity.