THE BIVALVES 



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the narrow end; but others exhibit a more or less elongate notch or slit on the 

 ventral or convex side. In some species of Dentalium the end has several notches, 

 and in Schizodentalium a series of holes takes the place of a ventral slit. Fossil 

 toothshells are numerous from the Devonian epoch upward, but are most abundant 

 in the Tertiaries. The living forms, of which about a hun- 

 dred have been described, occur in all parts of the globe, and 

 have been dredged at the greatest depths, although they are 

 probably most abundant in a few fathoms. 



THE BIVALVES Class Pelecypoda 



J> 



SECTION OF ANIMAI, OF 



Dentalium (enlarged). 



a. Front mantle cavity ; &. 

 Hinder mantle cavity ; 

 b. Mouth process ; c. 

 Anal opening ; d. Foot 

 cavity; e. Tentacle 

 supports ; f. Posterior 

 end. 



This great division includes all forms which secrete a 



bivalved shell, like the oyster, cockle, and mussel. The 



Pelecypods offer a different type of organization from that 



prevailing in the other classes. The absence of a head, and 



the bilateral symmetry of the animal, inclosed within a 



bivalved shell, are characteristic of the class. The mantle 



is divided into two similar lobes (), right and left, forming 



a flap on either side of the body, to which it is connected 



at the upper part beneath the hinge line of the shell. It is 



usually very thin, excepting at the edges, which are some- 

 times double or even threefold. In some genera the edges 



of the two lobes are free or unconnected at any point except- 

 ing at the dorsal attachment, in others they are joined in one 



or more places, leaving orifices for the protrusion of the foot 



(a) in front, and for 

 the entrance of the 



water to the gills (d, e) , and for the extrusion 

 of waste and other matter at the posterior 

 end (/). The mantle at this posterior 

 opening is often considerably produced, 

 forming one or two distinct tubes or siphons 

 (^), which vary considerably in length 

 in different groups, equalling in some in- 

 stances several times the length of the 

 shell. The extreme development of these 

 siphons obtains in Teredo, where they con- 

 stitute the principal mass of the animal. 

 Some pelecypods appear to be all foot, this 

 member being enormously developed in the 

 razor shells {Soleri), for example. In others 

 it assumes smaller dimensions, or it may be 

 absent, as in the oyster. It is used either 

 as a means of locomotion, or for burrowing 



SOFT PARTS OF RIVER MUSSED (Anodonta}. in sand or mud, or perforating rocks, wood, 



