THE VERTEBRATA 149 



Let us now return to the lowest vertebrate of 

 all, which has a large notochord and no bones. 

 This is the Amphioxus, the Lancelet. Amphioxus 

 has no bones whatever, and no head, in the sense 

 in which we usually employ that term ; that is to 

 say, most of the structures which we see in the 

 vertebrate head are undeveloped. The peculi- 

 arities of the structure of Amphioxus are many. 

 Among them may be named the curious gills : 

 these form a sort of basket-work along the sides 

 of the throat, which at first sight bears little re- 

 semblance to the gills of fishes, and reminds us of 

 those of Ascidians. The gills lead also, as in 

 Ascidians, to another cavity, the Atrial chamber. 

 This basket-work is formed, however, by the sub- 

 division of the primary pairs of gills. These are 

 very numerous, ninety pairs being sometimes 

 named as the number. They cut up the wall of 

 the throat to such an extent, that additional sup- 

 porting bars are needed to strengthen it ; and, 

 by the formation of these, both in parallel and in 

 transverse directions to the primary partitions, 

 the " basket-work " is produced, as the growth of 

 the animal proceeds. 



The primitive nature of the notochord is, how- 

 ever, perhaps the most striking feature of Am- 

 phioxus. The chord passes to the front of the 

 animal's snout head it can hardly be called 

 instead of ending in the middle of the brain, as 

 in true vertebrates, for there is, indeed, no " brain " 

 of any extent to lie in front of it ; and the noto- 

 chord, together with the spinal cord itself, have 

 no other protection than a fibrous sheath. The 

 spinal column is thus entirely absent, except so 

 far as it may be regarded as represented by this 

 thin sheath. 



