Bilaterally symmetrical coelomate Metazoa, in which the neural surface of the body is dorsal, the haemal ventral, i.e. the surface of locomotion, the reverse of what obtains in the other phyla of this division of Metazoa. The body is segmented, except in Urochorda, where segmentation is obscurely indicated in the caudal region alone. The central nervous system is formed typically from a neural (medullary) plate of epiblast which t y becomes converted into a neural (medullary) groove, and finally by the meeting and closure of the two edges of the groove, into a neural (medullary) tube. The dorsal wall of the archenteron gives origin to an axial rod of cells - the notochord - which primitively underlies the central nervous system in its whole extent, or, as in Urochorda, only in its caudal section. The mouth is anterior, and the stomodaeum on the ventral side of the nervous system, which it does not perforate, as in other Coelomata. The pharynx, or first section of the archenteron, immediately following the stomodaeum, is perforated by one or more pairs of lateral clefts developed as outgrowths of the hypoblast, coming into contact with the epiblast which then thins away and leaves an opening.

These clefts are respiratory in function, and vascular channels run in the partitions or septa between them. The anus is a ventrally placed proctodaeum, or, as in some Amphibia, Ceratodus and Petromyzon, a persistent blastopore. The coelome is either a schizocoele or crypt-enterocoele (p. xxx), formed in the two plates of mesoblast lying one on either side the notochord, as in Vertebrata, or an enterocoele, as in Cephalochorda, derived from two lateral outgrowths of the archenteron which divide from before backwards into a series of pouches, or primitive somites, in open communication for some time with the archenteron.

The phylum includes three sub-phyla - Vertebrata, Cephalochorda, and Urochorda (= Tunicata).

The somites of the body are formed in Vertebrata as follows. The dorsal section of the primitive coelome becomes separated from the ventral. The latter forms the permanent coelome, whilst the former is divided into a series of masses right and left lying on either side of the notochord. These masses are the somites, formerly known as primitive vertebrae. They give origin to the bodies of the vertebrae in part and to the muscle plates. The muscle plates extend both dorsally and ventrally, and give origin to the muscles of the body-wall, and in Elasmobranchii, Amphibia, and Lacertilia, of the limbs as well. The coelome extends round the archenteron, and its two halves meet ventrally and form a longitudinal septum. Traces of this septum persist as the ventral mesentery seen in a few Fish. The coelome in the region of the head becomes divided into a series of cavities by the formation of the mouth and the gill-slits. In Amphioxus, the first coelomic pouch belongs to the head; the remainder form the somites of the body.

Their dorsal sections give rise to the muscles.

After the formation of the notochord a small sub-notochordal rod of cells is developed from the dorsal aspect of the archenteron in Ichthyopsida among Vertebrata. It is doubtfully represented in the Chick. It is an evanescent structure, but is said to form the sub-vertebral ligament of Acipenser. It is interesting to note that a rod of cells derived from the neural aspect of the archenteron in Blatta and Clepsine forms a sheath for the nerve-cord.

The mouth of Urochorda is a stomodaeum. In Amphioxus it appears as a pore in the centre of an ectodermic disc-like thickening. Two views are taken as to its nature in Vertebrata. It is either a stomodaeum - the view taken later on, as there is certainly an ectodermic invagination - or it represents a modified pair of gill-clefts. In favour of this last-mentioned view, it is urged (1) that the fifth nerve branches over the dorsal angle of the mouth, just as e. g. the glossopharyngeal nerve does over the first branchial cleft; (2) that a branchial sense-organ lies at each dorsal angle from which the Gasserian ganglion is derived like the ganglia of the other segmental cranial nerves; (3) that in some Teleostei it appears as a double pit.

The part of the archenteron which occupies the tail in the larval Urochorda is metamorphosed partly into muscle-cells, partly into blood corpuscles. There is generally said to be a post-anal section of the archenteron in Vertebrata which communicates by a neur-enteric canal with the neural tube. But it is possible that the neural and archenteric tubes both open at the blastopore, as in certain Amphibia (Rana, Alytes, and Triton), where the blastopore certainly persists as anus; and that as the tail grows out, the blastopore is gradually shifted to the ventral surface, the neural tube still retaining its connection with it 1. This view ascribes a neural origin to the post-anal gut, and appears to explain its subsequent disappearance. See Spencer, Q. J. M. xxv: cf. Durham: Miss Johnson: Q. J. M. xxvi.

For the Hemichordata of Bateson, see Enteropneusta in the Vermes.