218 COMPARATIVE ANATOMY 



tion with the head. In the course of development the walls 

 of the head cavities are converted into muscles, the cavities 

 themselves disappear, and the primitive segmentation is almost 

 wholly obscured. The paired cranial nerves, however, give 

 evidence of the original segmental arrangement, and it is 

 interesting to note that the muscles of the eyeball are formed 

 from the walls of three of the embryonic head-cavities lying 

 in front of the auditory capsule. The whole question of the 

 segmentation of the head is still involved in obscurity, and 

 cannot be discussed here, but it should be borne in mind that 

 the embryo dogfish affords ample evidence of the forward 

 extension of myotomes into the head region, and greatly 

 strengthens the belief that the craniate vertebrates are de- 

 scended from an Amphioxus-like ancestor. There is still 

 much uncertainty as to the number of primitive myotomes 

 that enter into the composition of the vertebrate head, but 

 there is no doubt whatever that head - myotomes and 

 head - cavities exist. One must insist, however,, on the fact 

 that the primitive segmentation is lost in the cranium of 

 the adult. 



The trunk ccelom is divisible into two parts, the abdominal 

 and the pericardial cavities. The former is the large space 

 surrounding the abdominal viscera, and it should be remem- 

 bered that it communicates with the exterior by the abdominal 

 pores situated on either side of the cloacal aperture. The 

 pericardial cavity surrounding the heart lies in the ventral 

 wall of the throat just in front of the pectoral fins. Its roof 

 is stiffened by the basibranchial cartilage, its floor by the 

 anterior prominence of the pectoral girdle. It is divided from 

 the abdominal cavity by a membranous partition, but com- 

 municates with it by a narrow passage, the pericardio-peri- 

 toneal canal, running close below the oesophagus and dorsal 

 to the hepatic sinus. 



The alimentary canal is rather more than twice the length 

 of the distance separating the mouth from the cloaca, and 

 is divisible into buccal cavity, pharynx, oesophagus, stomach, 

 intestine, and rectum. The teeth have already been noticed. 

 They are borne in several concentric rows on the margins 

 of the upper and lower jaws, and are continually replaced 

 as they wear out by new rows developed in a fold of mucous 

 membrane behind the jaw. The teeth are simply imbedded 



