CONCHIFERA. 



Fig. 351. 



699 



contrary, the auricles do not receive the blood 

 until it has been exposed to the revivifying 

 influence of the organs of respiration. 



Of the organs of respiration. The whole 

 of the Conchiferbus mollusks respire by 

 means of branchiae (e, e,Jig. 346). These or- 

 gans are variously disposed according to the 

 form of the animal. They are symmetrical ; 

 and in almost all the genera there are two on 

 each side. The branchiae generally present 

 the form of membranous leaflets, of a qua- 

 drangular shape, though often unequal. They 

 are broad and short when the animal is glo- 

 bular, elongated and narrow when the animal 

 is lengthened in its general form. In the 

 greater number of genera the branchiae are 

 formed of two membranous layers or laminae 

 (a, b, Jig. 352) within the substance of which 

 the branchial vessels descend with great regu- 

 larity. In several genera, as the Archidae and 

 Pecten, the branchial vessels, instead of being 

 connected parallel to one another within the 

 thickness of a common membrane, continue 

 unconnected through their entire length, and 

 they are thus formed of a great number of 

 extremely delicate filaments attached by the 

 base within a membranous pedicle, in which 

 the branchial veins pursue their way towards 



Fig. 352. 



the auricle. In a great many families and 

 genera the branchiae of one side have no com- 

 munication with those of the opposite side ; 

 in some others however, as in the genus Unio, 

 the four branchial laminae meet under the foot, 

 and the whole of their vessels empty them- 

 selves into a venous sinus of considerable 

 size. 



2 z 2 



