446 MANUAL OF ZOOLOGY. 



nervous centres become prae - cesophageal, whilst others are 

 post- cesophageal. 



Furthermore, the floor of the "primitive groove" in the 

 embryo of all Vertebrates has developed in it at an early 

 period the structure known as the " notochord " or " chorda 

 dorsalis " (fig. 238, B, ch). This structure, doubtfully repre- 

 sented in any Invertebrate, is a semi-gelatinous or cartilaginous 

 collection of cells, forming a rod-like axis, which tapers at both 

 ends, and extends along the floor of the cerebro-spinal canal, 

 supporting the cerebro-spinal nervous centres. In some Ver- 

 tebrates, such as the Lancelet (Amphwxus), the notochord is 

 persistent throughout life. In the majority of cases, however, 

 the notochord is replaced before maturity by the structure 

 known as the " vertebral column " or " backbone," from 

 which the sub-kingdom Vertebrata originally derived its name. 

 This is not the place for an anatomical description of the 

 spinal column, and it is sufficient to state here that it is essen- 

 tially composed of a series of cartilaginous, or more or less 

 completely ossified, segments or vertebra, arranged so as to form 

 a longitudinal axis, which protects the great masses of the 

 nervous system. It is to be remembered, however, that all 

 Vertebrate animals do not possess a vertebral column. They 

 all possess a notochord ; but this may be persistent, and in 

 many cases the development of the spinal column is extremely 

 imperfect. 



Another embryonic structure which is characteristic of all 

 Vertebrates, is found in the so-called " visceral arches " and 

 " clefts " (fig. 239, D). The "visceral arches" are a series of 

 parallel ridges running transversely to the axis of the body, 

 situated at the sides of, and posterior to, the mouth. As devel- 

 opment proceeds, the intervals between these ridges become 

 grooved by depressions which gradually deepen, until they 

 become converted into a series of openings or " clefts," where- 

 by a free communication is established between the upper part 

 of the alimentary canal (pharynx) and the external medium. 

 In Fishes and many Amphibians the greater number of the 

 visceral clefts remain open throughout life ; and the visceral 

 arches of all fishes (except the Lancelet) throw out filamentous 

 or lamellar processes, which receive branches of the aorta and 

 constitute branchiae. In the higher Vertebrata all the visceral 

 clefts become closed, whilst no branchiae are ever developed 

 upon the visceral arches. 



The limbs of Vertebrate animals are always articulated to 

 the body, and they are always turned away from the neural 

 aspect of the body. They may be altogether wanting, or they 



