582 THE INVERTEBRATA 



tant than the visceral arteries. The veins from the foot and the vis- 

 cera join to form a pedal sinus and this opens into the vena cava. The 

 junction of these is marked by a sphincter muscle (Keber's valve). 

 This sphincter is closed when the foot is extended. The relaxation of 

 the muscles and the pumping of the blood into the sinuses of the foot 

 bring about the swelling of the foot. When the foot is retracted the 

 blood is largely contained in spaces in the mantle. The pallial cir- 

 culation is maintained during movement when the visceral circulation 

 is interrupted as described above. 



While the Protobranchiata have a nervous system with four distinct 

 pairs of ganglia (Fig. 373 D) in the remainder of the class the number 

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

 (Fig. 398 B). 



The sexes are usually separate in the Lamellibranchiata, but some 

 species of Ostrea and Pecten are always hermaphrodite, while this 

 condition is frequent in Anodonta. In the Protobranchiata the gonad 

 discharges into the kidney, but in most forms there is a separate 

 generative aperture. While most marine forms and the freshwater 

 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. 373 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 labial 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 



