500 



CHORDATA. 



it divides into several curved lines (fig. 559, SI). It is caused by a groove or a 

 canal beneath the scales which opens to the exterior by numerous canals through 

 the scales. The special sense organs (neuromasts) lie in the epithelium of the 

 canals. They may also occur in special pits in the skin (ampullae of Lorenzini, 

 Savi's -vesicles of the elasmobranchs), or they may occur separated in several 

 rows (Amphibia). Those of the head are innervated by branches of the facial 

 nerve, those of the trunk by the latcralis branch of the vagus These nerves 

 have their cerebral connections in close association with the auditory nerve, 

 which would imply a similarity of function Since these organs are lacking in 

 mammals and man it is difficult to determine their function. Experiment 

 seems to show that they are for the preception of water cu-rents and indirectly 

 the proximity of solid bodies. Mention has already been made of the nerve 

 end buds (taste) which, besides occurring in the skin, are especially abundant in 

 the mucous membrane of the mouth as well as on the lips and barbels 



The alimentary tract is spacious only in the oropharyngeal region. 

 Then it narrows to a slightly convoluted tube in which, the various regions 



A. 



FIG. 550. Digestive tracts of (.1 ) Squatina vulgar is (partly opened) and (5) Tra- 

 iui* riuliutns (from Gegenbaur). a/>, pyloric aeca; c, rectum; d, bile duct; dp, 



duct of air bladder; /, intestine; oe, oesophagus; p, pylorus; v, stomach; -vs, spiral gland; 



x, rectal gland. 



are not sharply marked off from each other. Mouth and pharynx fre- 

 quently bear teeth. In the teleosts the bones of the roof of the mouth and 

 the visceral arches may be covered with coalesced heckel-like teeth. In 

 the elasmobranchs the lining of the mouth, like the external skin, is 

 frequently covered with small dermal scales; the large chewing teeth are 

 confined to the edges of the Meckelian and pterygoquadrate arches, 

 where they are implanted in several rows behind one another. Since the 

 teeth are held only by membrane and are easily torn out, they may be 

 replaced indefinitely. Liver and spleen are always present ; pancreas and 

 gall bladder usually occur. In many fishes blind sacs, the pylorit cccca, 

 occur at the junction of stomach and intestine (fig. 550, B}\ others have 

 a spiral valve (A), a. fold of mucous membrane, which extends like a spiral 



