BIVALVES. 



401 



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

 tooth-shells are numerous from the Devonian epoch upwards, but are most abundant 

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

 hundred 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. 



This great division includes all forms which secrete a 



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



Pelecypods offer a different type of organisation from that 



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



the bilateral symmetry of the animal, enclosed within a 



bivalved shell, are characteristic of the class. The mantle 



is divided into two similar lobes (g), 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 



excepting at the dorsal attachment, in others they are joined 



in one or more places, leaving orifices for the protrusion of 



the foot (ft) 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 

 "\ 6 distinct tubes or siphons (/<), which vary 



considerably in length in different groups, 

 equalling in some instances several times 

 the length of the shell. The extreme 

 ,.'/ development of these siphons obtains in 

 Teredo, where they constitute 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 

 si nailer dimensions, or it may be absent, 

 -p as in the oyster. It is used either as a 

 means of locomotion, or for burrowing in 

 sand or mud, or perforating rocks, wood, 

 and other substances; its form conse- 

 quently being very variable. The mouth 

 (b) is situated at the anterior end of the 

 body, at the upper front part of the foot, 

 soft-parts ok rivbr-mussel (Ana ita). forming a simple transverse aperture. The 



vol. vi. — 26 



SECTION OF ANIMAL OF 



Dcntalium (enlarged). 



a, Front mantle - cavity ; d, 

 Hinder mantle- cavity ; 

 b, Mouth process ; c, 

 Anal opening ; d, Foot- 

 cavity ; e, Tentacle sup- 

 ports ; /, Posterior end. 



