3 



THE POPULAR SCIENCE MONTHLY. 



These bivalves possessing a foot present three pairs of ganglia in- 

 stead of two the anterior or oval, the posterior or branchial, and the 

 inferior or pedal. It occasionally happens, however, that the ganglia 

 of the posterior or of the inferior pair become approximated or even 

 fused into one. The fusion of the posterior pair takes place, as in the 

 oyster, when the branchiae from which they receive nerves 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 only by a short commissure, as in the mussel (Fig. 9, b). 



Fig. 8. Nervous System of the 

 Oyster. 



Fig. 9. Nervous System of the Common Mcssel. 



The separate existence or fusion of the inferior or pedal ganglia 

 depends upon the size and shape of the foot. The nerves in relation 

 with these ganglia are distributed almost wholly to this organ and its 

 retractor muscles. Where the foot is broad the ganglia remain sep- 

 arate, 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. ,p). 



Some of the special senses are unquestionably represented among 

 these headless Mollusca, though the distribution of the different or- 

 gans is very peculiar. Thus in Pecten, Pinna, Spondulus, the oyster, 

 and many others, very distinct and often pedunculated ocelli are dis- 

 tributed 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 circurupallial nerves. In the razor-fish, 



