CONCHIFERA. 



245 



In some genera the gills subserve a third purpose ; the oviducts open 

 into the dorsal channels, and the eggs are received into the gill-tubes and 

 retained there until they are hatched. In the river-mussel the outer gills 

 only receive the eggs, with which they are completely distended in the 

 winter months (Fig. 171, 0,0}. In Cyclas the inner gills form the mar- 

 supium, and only from 10 to 20 of the fry are found in them at one time ; 

 these remain until they are nearly a quarter the length of the parent.* 



Fig. 171. River-mussel. (Anodon cygneus ? )t 



The valves of the Conchifera are bound together by an elastic ligament, 

 and articulated by a hinge furnished with interlocking teeth. The shell is 

 closed by powerful adductor muscles, but opens spontaneously by the action 

 of the ligament, when the animal relaxes, and after it is dead. 



Each valve is a hollow cone, with the apex turned more or less to one 

 side ; the apex is the point from which the growth of the valve commences, 

 and is termed the beak, or umlo (p. 37.) The beaks (umbones} are near the 

 hinge, because that side grows least rapidly, sometimes they are quite mar- 

 ginal ; but they always tend to become wider apart with age. The beaks are 

 either straight, as in Pecten ; curved as in Venus ; or spiral, as in Isocardia 

 and Dicer as. In the latter case each valve is like a spiral univalve, especially 

 those with a large aperture and small spire, such as Concholepas ; it is the 

 left valve which resembles the ordinary univalve, the right valve being a 

 left-handed spiral like the reversed gasteropods. When one valve is spiral 

 and the other flat, as in Chama ammonia (fig. 185), the resemblance to an 

 operculated spiral univalve becomes very striking (see p. 47). 



* Some other particular respecting the organization and development of bivalve 

 shell-fish are given in the introductory chapter. For an account of their vascular 

 system see Milne-Edwards, An. Sc. Nat. 1847, Tom. VIII. p. 77. 



t The valves are forcibly opened and the foot (/) contracted ; a, anterior adductor- 

 muscle, much stretched ; p, p, palpi; g, inner gills; o, o, outer gills distended with 

 spawn ; 6, b, a bristle passed through one of the dorsal channels. 



