The amnion is absent altogether, and the allantois, when represented, never extends beyond the body-walls as a foetal membrane. Other important characters are the following.
The epidermic exoskeleton is either absent, or takes the form of enamel coating a subjacent dermic exoskeleton of bone. The cells of the epidermis are connected by protoplasmic processes to one another. The cutis is composed of vertical and horizontal connective tissue bundles as in Reptilia. There is a series of epidermic sense-bodies or nerve-eminences forming 'the system of the lateral line' - -which may be aborted in the adult. The integument forms a median fold extending along the back, round the tail, and as far forwards as the anus, very rarely in front of it. This fold is the azygos fin. It may persist as a continuous fold; be broken up into sections, the dorsal, caudal and anal fins; or else be aborted. The chondrocranium is often large. The basisphenoid bone is either absent or inconspicuous. Of the investing bones the parasphenoid, when present, is large. There are at least four pairs of branchial arches present in development, and the number may be greater. They are either retained throughout life, or considerable remnants are generally to be found in the adult. The notochord may or may not persist wholly or in part. There is no costal sternum, i. e. one formed from the ventral ends of the primitive ribs.
The spinal accessory nerve is a branch of the vagus: the hypoglossal represented by the first, or first and second spinal nerves. The digestive tract either ends in a cloaca, or the anus opens in front of the urino-genital apertures. The heart consists of a sinus venosus, one or two auricles, a ventricle and a conus arteriosus - the last-named being aborted in Teleostean Fish. A pair of aortic arches at least persists: and of the others present in development all or a certain number are temporarily or permanently connected with gills. The haematids are large, oval and nucleated. Branchiae external or internal are present either throughout life or during its first stages. The ova are generally numerous.
There are three classes, Amphibia, Pisces and Cyclostomi.
The organs of the lateral line consist of (I) pyriform sense-cells terminated at their outer ends by a single sense-hair, at their inner by a nerve-fibre; (2) of supporting cells either surrounding or mixed with (1). The sense-cells are shorter than the supporting-cells, and in this point nerve-eminences differ from the terminal buds of the skin of Teleostei where the two kinds of cells are of equal length. The nerve-eminences are circular, oval, or ridge-like, and the ridges may be almost continuous with one another. They are either freely exposed, and then their surface is protected by a delicate cuticle perforated by the sense-hairs: or they are surrounded by a delicate hyaline tube as in Amphibia and some adult Teleostei-. or they are sunk in furrows or canals which have apertures leading upwards from spot to spot, as in some Pisces. When thus sunk, the dermal skeleton generally comes into relation with them. In the region of the head they are supplied by the fifth nerve (and by the seventh in the Skate): on the body by a branch of the vagus, the lateral line nerve.
They appear to be segmentally arranged in the embryo in many instances, and it has been recently pointed out that a single eminence is seated at the dorsal end of each branchial cleft, and of the mouth at an early period. Hence the name 'branchial sense-organ' applied to them in this region. It is possible that the olfactory mucous membrane is derived from such an organ. The eminences multiply by division. In a few instances they are found scattered over the whole body, e.g. Mugil(see p. 85), a condition perhaps to be regarded as primitive.
Nerve-eminences, etc., Merkel, Endigungen der Sensibeln Nerven in der Haut der Wirbelthiere, Rostock, 1880; Leydig, Festschrift zur Feier des 100-jahrigen Bestehens der Natf. Gesellsch. in Halle, 1879. Branchial sense-organs, Beard, Q. J. M. xxvi.
On the origin of azygos fins, see p. 101, under interspinal bones.