334 THE ANIMAL KINGDOM. 



becomes converted into a neural (medullary) groove, and finally by the 

 meeting and closure of the two edges of the groove, into a neural (me- 

 dullary) tube. The dorsal wall of the arcnenteron gives origin to an axial 

 rod of cells the notochord which primitively underlies the central ner- 

 vous 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 de- 

 veloped 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 ( = Tunic ata). 



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 longi- 

 tudinal 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 Adpenser. 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 



