LAMELLIBRANCHIATA 507 



descends one side and ascends the other. The ascending vessels join 

 to form a longitudinal efferent vessel, which discharges into the longi- 

 tudinal vein of the kidney. Into this longitudinal vein is collected the 

 blood from the kidney network in general and by this channel blood 

 is returned into the auricle. It will be seen that the branchial cir- 

 culation is not important in Mytilus ; in Anodonta (Fig. 372) it is more 

 developed. 



While the Protobranchiata have a nervous system with four distinct 

 pairs of ganglia (Fig. 349 D) in the remainder of the class the number 

 is reduced to three by the fusion of the cerebral and pleural ganglia 

 (Fig- 373 B). 



The sexes are usually separate in the Lamellibranchiata, but some 

 species of Ostrea and Pecten are always hermaphrodite, while this con- 

 dition is frequent in Anodonta. In the Protobranchiata the gonad dis- 

 charges into the kidney, but in most forms there is a separate genera- 

 tive aperture. While most marine forms and the heshwater Dreissensia 

 have trochosphere and veliger larvae, some lamellibranchs incubate 

 the embryos within the ctenidia, and in the family Unionidae, which 

 includes Anodonta., the larvae are much modified (Glochidium). When 

 they are ripe the mother liberates them if a fish swims near her, 

 and they attach themselves to the gills or fins and become encysted 

 there. After a parasitic life which varies greatly in length they escape 

 from the cyst as young mussels. 



Order PROTOBRANCHIATA 



The best-known representative is Nucula (Fig. 349 D). It has a shell 

 of very characteristic appearance with numerous teeth on the hinge 

 line and a foot which, when fully extended, has a flat ventral surface 

 which has been compared with that of the gasteropod. But instead of 

 creeping by means of it the animal uses it for burrowing ; it is folded 

 up (as is seen in the diagram), and thrust into the mud, then opened 

 out and used as a holdfast, and the contraction of the retractor muscles 

 draws the body below the surface. While the surface of the ctenidium 

 is so small that the organ is of little use for feeding, the lahial palp is 

 enormous and divided into three parts. One of these is a kind of 

 proboscis which is thrust out of the shell and collects food by ciliary 

 currents. This is sorted and forwarded to the mouth by the other two 

 parts without the intervention of the ctenidium. 



The nervous system has distinct cerebral and pleural ganglia and 

 the gonads have retained their original connection with the kidneys. 

 These and some less important characters show that Nucula and its 

 relations are probably the most primitive of living lamellibranchs. 

 The specialization of the labial palps has had as its consequence the 

 partial suppression of the ctenidia, which remain in an undeveloped 



