EVOLUTION 11g 
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 
