MOLLUSC A. 51 



The arrangement of the puhnonary veins (Fig, 25) are 

 so easily made out, and so simple, that it is a very fine 

 illustration of the relations of the arterial and venous sys- 

 tems. These essentially consist of two parts, which I have 

 described as resembling two bushes with their branching 

 heads connected by a spongy mass of interlacing vessels in 

 the body of the oyster, and in some parts of the snail, and 

 in the gills by tubes. 



The general course of the arteries and veins is shown in 

 Fig. 27, Paludina, a water-breathing form ; the arteries 

 and the ventricle of the heart are shaded ; the veins, the 

 branchial tubes, and the cavity of the body in which the 

 blood collects, are left blank. The fluid passes from the 

 foot cavity into the opening of the large vein (>^'), and from 

 other veins {k) into the large vein (/^')» and through the 

 branchiae to the vein (/), and then to the auricle. This, 

 of course, opens into the ventricle {vr), and begins its 

 redistribution in the arteries (art). In order to expose 

 the internal organs, more than one animal is necessary. 



Open the foot of the snail (Fig. 25) by splitting along 

 the middle Hne of the bottom. Before the thick, muscular 

 layers are cut through, the yellow double line of the mucous 

 gland {gd) will be seen. The large mass of the foot gan- 

 glion («'), with its radiating branches, next comes into 

 view in the interior. The branches from this leading for- 

 ward to other ganglia {?i) above the throat are also visible. 



On the right side is the swollen bag {sac) containing the 

 dart; and, attached to this, the long penis {pti) with its 

 ducts ; and on the other side of these the oviducts {ov) 

 leading to the ovaries, — a complicated apparatus, which 

 we have not space to describe. 



The snail is, therefore, both perfect male and perfect 

 female, but self-impregnation does not take place. Two 

 individuals, as a rule, conjugate in the production of off- 

 spring, and mutually impregnate each other. 



The black tubes of retracted tentacles are also visible, 

 but better seen from above, as described farther on. The 



