CHAP. IV.] THE NERVOUS SYSTEM OF MOLLUSKS. 75 



The fusion of the posterior ganglia takes place, as in the 

 Oyster (fig. 22, b), when the branchiae from which they 

 receive nerves (c, c) come close together posteriorly. On 

 the other hand, in those Mollusks in which the branchiae 

 are farther apart, the two ganglia remain separate and are 

 connected by a short commissure, as in the Common 

 Mussel (fig. 23, 6). 



The separateness or fusion of the inferior or ' pedal ' gan- 

 glia depends upon the size and shape of the foot, since 

 the nerves in relation with them are distributed almost 

 wholly to this organ and its retractor muscles. Where the 

 foot is broad the ganglia remain separate, and are merely 

 connected by a commissure. But where the foot is small 

 and narrow, as in the Mussel, the two ganglia become 

 fused into one (fig. 23, p). 



Some of the special senses are unquestionably repre- 

 sented amongst these headless Mollusks, though the 

 distribution of the different organs is very peculiar. 

 Thus in Pecten, Pinna, Spondylus, Ostrsea, and many 

 other genera, very distinct and often pedunculated ocelli are 

 distributed over both margins of the 'pallium' or mantle. 

 These vary in number from forty to two hundred or 

 more, and are in connection with distinct branches of the 

 circumpallial nerves. In the Razor-fish, Cockle, Venus, 

 and other bivalves possessing prolongations of the mantle 

 known as 'siphon-tubes', the ocelli are situated either at 

 the base or on the tips of the numerous small tentacles 

 arranged round the orifices of these organs. And these 

 parts, in such bivalves as live in the sand, are often the 

 only portions of the body which appear above the surface. 

 The margins of the mantle are also garnished by a number 

 of short though, apparently, very sensitive tentacles, in 

 which the creature's most specialized sense of touch seems 

 to reside. 



