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 

 s- -Elasmobrancliii, including Sharks and Rays, Teleostomi, in- 

 cluding tlu- 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 



e 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 



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 si/e. Almost equally large is the ferocious Car char odon, 

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

 I Toad 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 

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



Mi->ides the typical sharks (Selachii) the group includes the skates 

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

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

 trom above downwards, the tail region is comparatively degenerate, 

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

 i >rming the greater part of the whole body. The ordinary gill-clefts 

 "l"' n " n tll(1 |""'T sin-lace of the body but the spiracle, which 

 r the indraught of the water of respiration, is situated on 

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



ln l! ' spindle-shaped muss of muscle on each side of the 



