CHAPTEE IV. 



MODIFICATIONS OF THE SKELETON. 



The lowermost sub-class of fishes, whicli comprises one 

 form only, the Lancelet {BrayicMostoma [s. Ayn/pJiiosiyiis] lanceo- 

 latum, possesses the skeleton of the most primitive type. 



Fig. 28. — Branchiostoma lanceolatum. 

 a,, Mouth ; 5, Vent ; c, abdominal porus. 



The vertebral column is represented by a simple chorda 



Fig. 29.— Anterior end of body of Branchiostoma (magn.)' 



d, Chorda dorsa.lis ; e, Spinal chord ; /, Cartilaginous rods ; g, Eye ; 



h, Branchial rods ; i, Labial cartilage ; Jc, Oral cirrhi. 



dorsalis or notochord only, which extends from one extremity 

 of the fish to the other, and, so far from being expanded into 



