14 



COMPARATIVE ANATOMY 



CHAP. 



CLASS IV. Lamellibranchia Pelecypoda, Bivalva, Acephala, Aglossa . Mussels. 



The body is symmetrical and more or less transversely flattened ; it has two 

 large lateral leaf-like mantle lobes, enclosing a spacious mantle cavity large enough 

 to contain the foot, which is usually hatchet- or wedge-shaped. The shell consists 

 of two lateral valves connected together only at the dorsal hinge. It is closed by means 

 of two adductor muscles passing transversely from one valve to the other (Dimyaria) ; 

 occasionally the anterior adductor degenerates and only one remains (Monomyaria). 

 On each side in the mantle cavity there is a ctenidium. There are no jaws, no 

 pharynx, no radula, no tentacles, and no distinct head. The kidneys and genital 

 organs are paired, and the latter either have separate ducts or eject their products 

 through the nephridia. The heart has two auricles. At each side of the mouth 

 there are two oral lobes. Either sexually separate or hermaphrodite. They live 

 in salt or fresh water, and are either limicolous or attached. 



ORDER 1. Protobranchia. 



The gills with two rows of leaflets, in the posterior part of the mantle cavity ; 

 they correspond in all respects with the ctenidia of the Zeugobranchia, their ends 



FIG. 21. Nucula nucleus, left aspect after removal of the left valve and mantle (after 

 Pelseneer). a, Anterior adductor ; 6, anterior retractor of the foot ; c, elevator of the foot; d, genital 

 mass ; e, hypobranchial gland ; /, posterior retractor of the foot ; g, posterior adductor ; h, cteni- 

 dium ; i, mantle cavity ; k, creeping sole of the foot (Q ; m, oral lobes (labial palps) with posterior 

 appendages n and o. 



project freely backward into the cavity. The foot has a sole for creeping. The 

 pleural ganglion can be distinguished from the cerebral. Fam. Nuculidae (Nucula 

 [Fig. 21], Leda, Yoldia, Solenomyidae). 



ORDER 2. Filibranchia. 



The branchial leaflets of the ctenidium have become lengthened out into long 

 filaments hanging far down into the mantle cavity. Each is in two parts, the proxi- 

 mal descending and the distal ascending (cf. Fig. 88 B). Fam. Anomiidse: mantle open 



