EVOLUTION 119 



but also as a leaping organ, and it subserves similar 

 ends in Trigonia, Donax, and a few others. 



In the case of those Bivalves that are inactive or 

 stationary like the borers (Pholas, Saxicava, Teredo, 

 etc.), or are attached, like the Oysters, some of the 

 Scallops, etc., the foot becomes rudimentary through 

 disuse ; indeed, in the adult Oyster it is entirely 

 wanting. 



Certain of the Scallops dart about by rapidly 

 opening and closing their valves ; Ensis and Soleno- 

 mya by expelling water from the posterior aperture 

 of the mantle and siphons. 



Of the movements of the little pelagic Planktomya 

 nothing is at present known. 



While Nautilus creeps about on its numerous series 

 of weak tentacles, the Cuttlefish crawl about and 

 catch their prey by means of their far more powerful 

 arms. Some of the latter swim leisurely through 

 the water by means of their fins ; but all, including 

 Nautilus, can dart rapidly backwards by expelling 

 the water from the pallial cavity through the funnel. 



Among the evolutionary developments called forth 

 by environmental requirements may be reckoned 

 those instances of protective resemblances and 

 coloration that are as prevalent in the Mollusca as 

 in the rest of the animal kingdom. 



The mottled markings on the shell of the common 

 Garden Snail play on a small scale the same part 

 that they do in the Giraffe, and serve to make the 



