FORM, AND MOVEMENT. 6$ 



terrestrial structure which have taken to the water approxi- 

 mate to the fish form. All stages are observable. Com- 

 pare, for example, with regard to loss of neck constriction, 

 reduction or loss of ear pinna, modification or loss of hind 

 limbs, the following series: Water Vole, Otter, Seal, 

 Dolphin or Whale. 



In all of these the neck is thickened, and there is practically no 

 constriction. In the water vole the ear pinna is covered with rather 

 long hair and lies flat against the head ; in the otter the pinna is 

 greatly reduced in size, and does not project upon the head ; in 

 seals, dolphins, and whales it is entirely absent. As regards hind 

 limbs, in the semi-aquatic forms these limbs are of normal size ; in 

 seals they are directed backward in the line of the body and are 

 most useful as swimming organs ; in the others entirely aquatic 

 mammals they are lost, save for vestiges concealed within the body. 



Surface. Though scaly, the body presents a smooth 

 surface. Mucus glands are present in the skin, and in 

 some fishes it is kept in an extremely slimy or slippery 

 state by the products of these glands. An excellent 

 example of this is the small " butterfish " so common in 

 the pools around our shores, although here the slipperiness 

 has the additional protective value of rendering the fish 

 extremely difficult of capture. It is worth while here also 

 comparing the "set" which is assumed by the hair upon 

 the aquatic mammals, e.g. otter and seal. 



Specific Gravity and Centre of Gravity. It is well 

 known that dead fish float. Their specific gravity is about 

 equal to that of water. Their centre of gravity is high up 

 on the body a little way behind the head. These points 

 might be verified on a dead fish. Bearing this in mind, it 

 will be readily seen that, were it not for, the action of the 

 fins, the balance of the fish would be easily upset. But 

 with balance maintained, the result of these arrangements 

 is that a minimum of effort is sufficient to effect movement. 

 The support given to the body is such that fish of this 

 shape do not require to lie down in resting. A little watch- 

 ing of the fish will help to make these things clear. 



