Physical characteristics

As the closest living relatives of vertebrates, cephalo-chordates occupy an eminent phylogenetic position. They are invaluable in reconstructing the evolutionary step from invertebrate to vertebrate along the lineage that leads to mammals and humans. Therefore, cephalochordates are of great interest to numerous researchers and consequently their anatomy is well known.

Notochord

Dorsal nerve cord

Dorsal aorta

D°r"sal fin Fin rays

Caudal fin

Dorsal nerve cord

Dorsal aorta

Caudal fin

Velum

Rostrum

Buccal cirri

Velum

Pharynx 1 Caecum

Pharyngeal gill slits

Anus Ventral fin Atriopore Myotome blocks Gonads

Lancelet anatomy. (Illustration by Christina St. Clair)

A median fin is present along the entire dorsal length of the animals, extending at the anterior end into a short rostral fin and at the posterior end a tail fin. The ventral fin runs between anus and atriopore. In front of the atriopore paired finlike folds, the metapleural folds, extend to the anterior region of the pharynx. The dorsal and ventral fins contain so-called fin chambers enclosing coelomic cavities and fin rays that consist of a substance that could serve as an energy store.

The epidermis consists of a single-celled layer, a primitive feature compared to the multi-cellular epidermis in vertebrates. It contains gland cells that secrete a thin sheet of mucus covering the surface of the animal. Different types of sensory cells are distributed in the epidermis, especially in the rostral region.

The dorsal nerve cord of cephalochordates has a central canal that is enlarged at the anterior end of the cord. This enlarged region is known as the cerebral vesicle and is homologous to parts of the vertebrate brain as indicated by morphological similarities (infundibular cells, Reissner's fiber) and similarities in the expression patterns of developmental genes. A peculiarity of the cephalochordate neural cord is the presence of numerous pigmented ocelli, the so-called pigment-

cup ocelli. In addition, several more structures in the neural cord are probably light sensitive, e.g., the Joseph cell receptors and the lamellar body in the cerebral vesicle. A photore-ceptor in the anterior cerebral vesicle of larvae shows significant similarities to the paired eyes of vertebrates and could be homologous to them. Outside the central nervous system one finds receptors (e.g., corpuscules de Quatrefage), especially in the epidermis of the buccal cirri, velar tentacles, the atrium, and along the metapleural folds.

The notochord extends from the anterior to the posterior end. It is a skeletal element that is the antagonist of the lateral muscle cells that attach to it. Curiously, it is itself an active tissue. It consists of specialized muscle cells and is innervated from the nerve cord. Vacuoles within and between the cells act as a hydroskeleton and help in adjusting the rigidity of the notochord. The notochord is the main skeletal element. Aside from it, in the pharynx, between the gill slits, the branchial bars contain skeletal, cartilaginous rods. In addition, similar rods support the anterior cirri.

A pattern of repeated V-shaped lines is visible on the side of a cephalochordate. It is similar to the pattern seen in a fish fillet, though simpler, and of course, minute. This pattern is caused by the connective tissue that separates individual muscle segments. The innervation of the muscle cells is quite different from the vertebrates. There are no ventral spinal nerves that innervate the cells as in vertebrates. Instead the muscle cells themselves form long thin extensions that approach the nerve cord and are innervated directly there.

It has long been believed that cephalochordates possessed excretory cells very similar to polychaetes. Electron microscopy showed that the similarity is superficial. The excretory cells of cephalochordates are unique in the animal kingdom though in evolutionary terms they are probably derived from the so-called podocytes that are found in other deuterostomes. Excretory organs (nephridia; sing. nephrid-ium) are found serially along the dorsal part of the pharynx. They are associated with blood vessels and function in a similar way as the vertebrate kidney. In addition to these nephridia a nephridium is situated just in front of the mouth (Hatschek's nephridium).

The general arrangement of blood vessels is similar to vertebrates, but the vessels are not lined by cells as in vertebrates. The region where a heart would be expected is contractile. In notable similarity with vertebrates a portal vein connects a capillary system around the posterior intestine with a capillary system around the hepatic caecum.

A pharynx with gill slits is common to all chordates: tuni-cates, cephalochordates, and the more primitive members of the vertebrates. In cephalochordates a vestibule lies in front of the pharynx that is guarded by the cirri. The vestibule contains ciliated tracts (wheel organ), an excretory organ (Hatschek's nephridium), and a shallow groove (Hatschek's pit) that is probably homologous to the adenohypophysis in vertebrates. The pharynx proper begins with the mouth opening that is surrounded by velar tentacles. The pharynx is perforated by up to 200 gill slits. Along its ventral midline runs a groove, the endostyle that produces mucus and thyroxine, indicating homology to the thyroid in vertebrates. The gill slits do not open into the open water, but the whole pharynx is covered by the outer body wall. In this way a space around the pharynx is created, which is called the atrium. The atrium opens in the ventral midline through the atriopore anterior to the anus.

The pharynx leads into the short esophagus and from there the gut extends straight posteriorly and opens through the anus. Ventrally, at about the junction of the pharynx and esophagus a blind ending caecum projects anterior. It is called the hepatic caecum and is probably homologous to the vertebrate liver.

Depending on species, adult animals range in size 0.4-3.2 in (1-8 cm) in total length. The color of all species is whitish to a creamy yellow, sometimes with a tint of pink. The mucus covering the epidermis can reflect iridescently.

For identification and taxonomic purposes the so-called myotome (muscle block) formula is used. Three numbers signify the number of myotomes anterior to the atriopore, between atriopore and anus, and posterior to the anus. In

A lancelet (Amphioxus) is seldom more than 3 inches (8 cm) long; it resembles a slender fish without head definition or eyes. (Photo by Animals Animals ©E. R. Degginger. Reproduced by permission.)

addition, the number of fin chambers in the dorsal fin serves as a diagnostic character. Epigonichthys differs from Branchiostoma in that it possesses only one series of gonads on the right side of the body, whereas the gonads are paired in Branchiostoma.

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