330 MANUAL OF ELEMENTARY ZOOLOGY 



the cavity, which behind is narrow like the central canal 

 of the frog's spinal cord, is in front wide like that of the 

 frog's brain. Below the central nervous system, along the 

 whole length of the body, lies an elastic rod, the notochord, 

 derived from the endoderm in the course of development 

 and bound to the nerve cord by a connective tissue sheath 

 which surrounds them both. In front it extends beyond the 

 nerve cord. There is no skeleton of bone or cartilage, but 



Fig. 238. — A, the hinder end of the pharynx of Ampkioxus, from the 

 left side, highly magnified ; B, a diagram of the mode of origin of 

 the gill clefts. 



est., Endostyle ; pr.b., primary bar ; syn. } synapticulse ; t.li., tongue bar ; 2°j/., the 

 two secondary gill slits which arise from a primary slit. The skeleton is 

 shown in black. 



stiff rods of organic material support the gill bars and 



cirri, and gelatinous " rays " the dorsal and ventral fins. 



The mouth opening leads into a wide cavity known as 



the pharynx, which forms about half the length 



Alimentary of the gut, and is placed in a portion of the 



KJwisoeiil Dodv which is enclosed by the atrium. Its 



oavity. sides are pierced by the gill slits, which lie 



obliquely, their lower ends being behind the 



upper, so that a number of them are cut in a transverse 



