268 MOVEMENT 



so as to give a fresh impetus when they are again 

 suddenly approximated. Meanwhile the fore limbs are 

 pressed against the thorax, and appear to be quite 

 inactive.* In the intermediate stage, when the legs 

 are incompletely developed, and the tail has not yet 

 disappeared (Fig. 188, third row), the batrachian has a 

 mixed mode of progression. Between the hind legs, 

 which execute the movements of swimming, the tail 

 keeps up an incessant wriggling motion. 



Snakes, Eels, Fish. — Snakes have a slightly different 

 method of progression according as they are on land 

 or in water. An ordinary adder placed in the dry 

 canal, before described, executes undulations of con- 

 siderable amplitude (Fig. 189). As the animal moves 

 along, the undulations pass from the anterior to the 

 posterior end of the body, the same as in the case of 

 the swimming eel. A water adder placed in the dry 

 canal progresses in the same manner, as also does an 

 eel. But when these same animals are placed in water, 

 they swim about with an undulatory movement of less 

 amplitude, but with far greater regularity. Fig. 190 

 represents an eel in the act of swimming ; the method 

 of progression is identically the same as that of the 

 adder, except that the movement of the tail is more 

 accentuated. The tail of an eel is transversely 

 flattened, and imparts a movement like that of other 

 fish ; the undulations of the tail are, however, more 

 pronounced than those of the rest of the body. 



Among other fish the undulations of the body are 

 less marked, although very noticeable in the case of the 

 dog-fish, which is a long-bodied animal (Fig. 191). It is 

 to be observed only in the tail in some species, the 

 movements of the body being but slightly developed, 

 as, for example, is the case with the Cyprinidae. 



* Owing to a mistake in the engraving, the order of the images has 

 been changed in several instances on the second line of figure. 



