142 THE BOOK OF NATURE STUDY 



cannot move except along with the whole floor of the mouth. 

 The first occurrence of a movable tongue is in the Amphibians. 



(/) Other points of interest in connection with the head 

 (i) The nostrils of fishes are altogether smelling-organs. They 

 have not anything to do with breathing. In almost all cases 

 they are without any connection with the mouth. In most 

 bony fishes, e.g. cod, there are two holes for each nostril. (2) 



G.G 

 l - p.v.f 



FIG. 50. The plaice Pleuronectes platessa in illustration of the study of shapes and 

 external characters. This flat-fish should be contrasted with the skate. It rests 

 and swims on its left side ; the skate on its ventral surface. In its swimming by 

 curving the body as a whole, including the tail the pectoral fins (P.V.f.) are quite 

 unimportant as far as progression is concerned ; in the skate the pectoral fins are 

 the organs of progression and the tail is unimportant. In the plaice the unpaired 

 fins above and below are strongly developed, again in great contrast to the skate. 

 The plaice has a bony skeleton, and its gill-clefts are overlapped by a gill-cover 

 (G.C.) ; the skate has a gristly skeleton, and its gill-cleft opens directly on the surface. 

 The young plaice are symmetrical, and swim like other young fishes, with the middle 

 dorsal line upwards. They live near the surface. As the body grows it becomes 

 unsymmetrical, as if it were forcibly compressed from side to side ; the fish ceases 

 to swim near the surface with the back up ; the left eye (L.E.) comes round to the 

 right side ; and the left side, turned away from the light, does not develop any pig- 

 ment. The figure shows also the lateral line (L.L ) and the pelvic fin (P.V.f.). 



The eyes are practically lidless. (3) Except in gristly fishes, 

 there is no external ear-opening or any external indication of 

 the position of the ear, which is, however, well developed. (4) 

 In gristly fishes, such as skate and dogfish, one of the gill-clefts 

 opens dorsally behind the eye, and is called the " spiracle." In 

 the main it aids the mouth, serving as an entrance for the water, 

 which passes out by the other gill-clefts. In some circumstances 



