372 MANUAL OF ZOOLOGY. 



lection 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 La-ncelet (Amphioxus\ 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 of 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. 153, 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 deve- 

 lopment 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. 



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 

 may be partially undeveloped ; but there are never more than 

 two pairs, and they always have an internal skeleton for the 

 attachment of the muscles of the limb. 



A specialised blood-vascular or "haemal" system is present 

 in all the Vertebrata, and in all except one the Amphioxus 

 there is a contractile cavity or heart, which never consists of 

 less than two chambers provided with valvular apertures. In 

 all the Vertebrata the heart is essentially a respiratory heart 

 that is to say, it is concerned with driving the impure or venous 

 blood to the breathing-organs ; and in its simplest forms (fishes) 

 it is nothing more than this. In the higher Vertebrates, how- 

 ever, there is superadded to this a pair of cavities which are 

 concerned in driving the pure or arterial blood to the body. 

 In the case of the Mammals, these two circulations are often 



