128 PRIMITIVE FORMS OF LIFE 



approach one another more and more, and eventually fuse in a 

 single tube pierced above by two openings. The Holothurian 

 thus takes on the shape of a bottle, whose neck carries the 

 two digestive openings that have now become contiguous. 1 



The Mollusca are at least as accommodating so far as posture 

 is concerned. The molluscs with spiral or helicoidal shells 

 crawl on their ventral surface ; but all those capable of 

 swimming, even if only temporarily, swim upside down, the 

 abdomen uppermost and the back below. Some molluscs, 

 indeed, are exclusively swimmers ; 2 others exclusively crawlers, 

 and others swim or crawl, according to circumstances. 



The molluscs with bivalve shells have aptitudes even more 

 varied. Mussels, and those molluscs akin to them, suspend 

 themselves by filaments which constitute the byssus ; the 

 Oysters, Pecten, Spondylus, etc., live resting on their sides ; 

 Tridacnae live on their backs on the polypary ; Venus, 

 Razor-shells, Pholades, and a host of others, bury them- 

 selves in the sand or penetrate into holes which they 

 hollow out even in rock, and live immobile, the head, or 

 what takes its place, being furthest in. A special form 

 of body corresponds to each of these attitudes, which is easily 

 accounted for by the continuous action of weight upon the 

 various internal parts of the immobile creatures. The body of 

 the mussel enlarges in the region turned downwards, 

 and becomes pointed in the neighbourhood of attachment of 

 the byssus ; the lower valve of oysters and other bivalves 

 that lie on their sides, originally symmetrical with the upper 

 valve, swells so as to form a sort of chamber, of which the 

 upper valve, flattened to concavity, is nothing more than a 

 cover ; the heavier organs of Tridacna sink below the 

 lighter, and reach almost to the hinge of the shell, so that the 

 mollusc appears doubled up inside it. In the species that 

 immobilize themselves in holes, the thick lime-secreting 

 mantle becomes elongated into two long siphons, one for 

 the entrance of the water which brings the animal air and food, 

 and the other for evacuation. These modifications, with the 

 exception of the last, have been brought about by the persistent 

 action of a most ordinary cause, namely weight pressure, which 



1 Rhopalodina (LIV, 280). 



2 Nautilus, Pteropods, Ianthina, and the larvae of all the marine Gasteropod 

 Molluscs. 



