IV. VERTEBRATA: PISCES. 565 



sense organs, which may be grouped in several lines or occur in pits (am- 

 pullae) in the skin in other places. Their function is obscure, since noth- 

 ing of the sort occurs in man or mammals. They are specific organs of 

 aquatic vertebrates and possibly have to do with the perception of water 

 pressure. 



The alimentary tract is spacious only in the oropharyngeal 

 region. Then it narrows to a tube in which the various regions 

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

 frequently bear teeth. In the teleosts the bones of the floor of 

 the cranium and those of the visceral arches may be covered with 

 coalesced heckel-like teeth. In the elasmobranchs the teeth are 

 mostly confined to the lower jaw and the pterygoquadrate, but are 

 in rows, the anterior row alone being functional; but as these are 

 loosely held they are easily torn out, when they are replaced by 

 the row behind. Liver and spleen are always present; pancreas 

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

 pyloric caeca, occur at the junction of stomach and intestine (fig. 

 593, B)', others have a spiral valve (A), a fold of mucous mem- 

 brane, which extends like a spiral stairway into the lumen of the 

 intestine, increasing the digestive surface. Caeca and spiral valve 

 rarely occur in the same fish. 



B 



FIG. 593. Digestive tracts of (A) Squatina vulgaris (partly opened) and (B) Tra- 

 chinus radiatiis. (From Gegenbaur.) ap, pyloric caeca ; c, rectum ; d, bile duct ; 

 dp, duct of air bladder; i, intestine ; oe, oesophagus ; p, pylorus ; i\ stomach ; vs, 

 spiral gland ; #, rectal gland. 



Gills of two types occur (fig. 594, A and B). In both the gill 

 clefts, which lie between successive branchial arches, begin by 

 openings in the pharynx, but differ in their external openings. 

 In the elasmobranch type (A) the external openings are a series of 

 slits separated by broad dermal bridges which cover the gills and 

 gill clefts (fig. 598). The gills are vascular folds of mucous mem- 



