508 



CONCHIFERA. 



(1336.) When we consider the position of the ovary in these bivalves, 

 placed as it is in the substance of the body, and reflect upon the immense 

 number of eggs to which they give birth (for thousands of ova are 

 generated by every one of these prolific beings), we perceive that, with- 

 out some special provision, the imprisoned animals would, when gravid, 

 be seriously inconvenienced and exposed to continual danger, as the inor- 

 dinate enlargement of the ovary would preclude the possibility of bringing 

 the valves of the shell in contact with each other. In order to obviate 

 the difficulty referred to, the ova are expelled from the ovarian nidus 

 in an immature condition, and complete their growth in a situation 



Fig. 255. 



1. Oyster (Oetrea edulis), showing the ramifications and excretory duct of the generative 

 system. 2. Spermatic filaments. 3. Secerning culs de sac from an individual almost entirely 

 male. 4. Spermatozoids, magnified. 5. Ovum enveloped in its capsule. 



where, being diffused over a larger surface, the shells may be closely 

 approximated ; and, moreover, the eggs and their contained offspring 

 are by this contrivance freely exposed to the influence of the medium 

 around, so as to allow a kind of respiration to be enjoyed by the un- 

 hatched young. The situation chosen is the branchial fringes, over 

 which the imperfect spawn, or spat, as it is technically termed, is found 

 widely spread towards the close of gestation, still retained beneath the 

 shelter of the shell of the parent, and thus preserved from destruction, 

 but at the same time, being in such a position freely washed by the 

 ciliary currents, the respiration of the included embryo is adequately 

 provided for. 



(1337.) In the large branchial laminae of the freshwater Mussel, it 

 is to be remarked that both pairs consist of an intertexture of vessels 

 arranged in a rectangular lattice-work, and covered by a delicate mem- 

 brane, whilst the two external are distinguished by a structure which 



