348 ZOOLOGY FOR MEDICAL STUDENTS chap. 



function of contraction has been reduced while the primitively subsidiary 

 role of producing electric disturbance has become predominant. 



The typical fishes fall naturally into three sharply defined main 

 groups — Elasmobrancliii, including Sharks and Rays, Teleostomi, in- 

 cluding the vast majority of fish, and Dipnoi or Lung-fish. 



Of the Elasmobranchii the structure of a typical example has been 

 described in detail in Chapter IX. The main features which together 

 serve to mark off the Elasmobranchs from other fishes are : (i) the placoid 

 scales, (2) the restriction of bony tissue to the scales — the main skeleton 

 being entirely cartilaginous, (3) the separate gill-openings, (4) the absence 

 of a lung or air-bladder, (5) the persistent external opening of the otocyst, 

 (6) the large richly-yolked eggs. With these are associated two important 

 features common to other archaic types of fish, namely the presence 

 of a spiral valve in the intestine, and the presence of a rhythmically 

 contractile conus arteriosus provided with pocket-valves. 



Apart from interesting extinct types the group includes the modern 

 sharks and dogfish. The largest of all fish are the Basking-sharks or 

 Sail-fish {Selache maxima) which reach a length of 35-40 feet and, like 

 the whales among mammals, feed on small pelagic animals. These fish 

 are provided with greatly elongated horny gill-rakers set close together 

 like the teeth of a comb and forming an efficient mechanism for straining 

 off food-organisms from the water'as it rushes out through the gill-clefts. 

 In correlation with this peculiar method of feeding the teeth are much 

 reduced in size. Almost equally large is the ferocious Carcharodon, 

 which reaches a length of over thirty feet. In this case the teeth are 

 broad flat triangular blades with finely serrated edges. Judging from 

 the size of occasional teeth dredged up from the bottom of the Pacific 

 and others from recent geological deposits it would appear that sharks 

 of this genus have attained in the past a length of as much as ninety 

 feet. 



Besides the typical sharks (Selachii) the group includes the skates 

 and rays (Batoidei) modified in accordance with their habit of swim- 

 ming along the sea-bottom. In them the body is much flattened 

 from above downwards, the tail region is comparatively degenerate, 

 while on the other hand the pectoral fins are enormously enlarged, 

 forming the greater part of the whole body. The ordinary gill-clefts 

 open on the lower surface of the body but the spiracle, which 

 serves for the indraught of the water of respiration, is situated on 

 the dorsal surface so that mud is not drawn in with the respiratory 

 current. 



In the skates a spindle-shaped mass of muscle on each side of the 



