158 



ORGANIZATION OF THE HEAD 



1 1 . The gut of elasmobranchs 



The digestive system of sharks shows several changes from the plan 

 found in lampreys, especially the presence of a true stomach, charac- 

 teristic of all gnathostomes. Apparently little or no digestion goes on 

 in the mouth and pharynx. The teeth consist of rows of backwardly 

 directed denticles (Fig. ioi). They are carried on special folds of skin 

 lining the jaws and are continually replaced as they are worn away on 



Fig. ioi. Sections through the jaws of a, dogfish (Scyliorhinus), and n, sand-shark 



(Odontaspis), showing the transitions between dermal denticles (d) and teeth (t). 



(From Norman, partly after Gegenbaur.) 



the edge. The replacement of milk by permanent teeth in mammals 

 is a relic of such serial replacement in a fish. The 'gill rakers' arc 

 rods attached to the branchial cartilages and serving to prevent 

 the escape of prey. The basihyal supports a short non-protrusible 

 tongue. 



The wall of the pharynx is lined by a stratified epithelium on to 

 which open numerous mucous glands, sometimes complex. The mucus 

 serves to assist the passage of the food, but probably has no strictly 

 digestive function, though the salivary glands of higher vertebrates 

 no doubt originate from a modification of these mucous glands. 



The pharynx narrows to an oesophagus with thick muscular walls, 

 leading without sharp transition to the stomach. We have seen that in 

 cyclostomes the oesophagus opens directly into the region of gut that 

 receives the bile and pancreatic secretion. The stomach, which we 



