4 6 ANIMAL LIFE PAST AND PRESENT. 



recognised by their oval, flat, and boat-like form, some 

 of the species attaining a large size. Most of them 

 swim entirely by the aid of their hind legs, which are 

 greatly enlarged, flattened, and fringed. In the curious 

 little " Whirligig" Beetles (Gyrinus), which are so often 

 seen performing their mazy evolutions on the surfaces 

 of ponds and rivers, the reverse of this arrangement 

 obtains, the front pair of legs being enormously 

 elongated, and the second and third pairs very short 

 and paddle-like. The forward motion of these beetles 

 is produced by these short paddles, while the curves 

 are formed by the long fore limbs, which are darte 1 

 out first from one side and then from the other, so as 

 to change the direction of the body. 



The last Invertebrate group we have to mention is 

 the large one of the Molluscs, or Shell-fish. Here by 

 far the greater number of species are aquatic, and 

 breathe by gills, so that we may regard those which 

 are swimmers as being primitively so. Although the 

 adults of the Bivalve Molluscs are either fixed to some 

 solid substance (Oysters), or are merely capable of 

 leaping or turning (Cockles and Fan-shells), yet in 

 their young state all these Molluscs are free swimmers, 

 young oysters being provided with swimming-organs 

 composed of delicate hairs. It thus seems probable 

 that these locomotive habits have been transmitted 

 to the young bivalves from originally free-swimming 

 ancestors. 



The ordinary Sea Snails (Gastropods), in which the 

 adult creeps on solid surfaces by means of its greatly 



