(1331). The ovary is generally a wide glandular sacculus, occupying a considerable portion of the visceral mass. In the Oyster, when full of spawn, it is largely spread through the body; and if at such seasons its delicate walls are ruptured, countless ova of microscopic dimensions escape from the lacerated part. In Pecten the ovary is very conspicuous, from the brilliant colour of the eggs contained in its interior; it constitutes the greater part of the bulk of that prominent tongue-like organ which projects between the branchiae (fig. 247, f): or in genera where the foot is very largely developed, as in Cardium rusticum, a great part of the base of that organ is hollowed out into a capacious cavity, enclosed by its muscular walls, wherein the delicate folds of the ovarium (fig. 253, a) are partially imbedded, together with a portion of the intestinal canal (c).

Cardium: a, oral orifice; b, foot.

Fig. 252. Cardium: a, oral orifice; b, foot.

* Ann. des Sci. Nat. n. s. torn. ix.

(1332). In almost all the Lamellibranchiate A-cephala there is situated on each side of the body, near the insertion of the branchiae, between the abdomen, the posterior muscle of the valves, the heart, and the liver, a gland, of a brown colour, which, from its discoverer, has received the name of the organ or sac of Bojanus*, the nature of which has long been a puzzle to comparative anatomists. It seems to be intimately in relation with the reproductive apparatus, the excretory canals of which always open either into its interior or in its immediate vicinity. This organ is always readily recognizable. On separating the branchial lamella) after placing the bivalve upon its back (that is to say, upon that part of its circumference which corresponds with the hinge), the student will observe on each side of the visceral mass an oblong body of variable tint, the shape of which depends more or less upon that of the animal.

Cardium rusticum.

Fig. 253. Cardium rusticum.

* "Mem. sur 1 Organe de Bojanus," par Dr. II. Lacaze-Duthiers (Ann. des Sc. Nat. 1855).

(1333). The structure of this gland is rather complicated, its interior being made up of numerous cavities communicating with each other. Its relations with the generative system present themselves under three aspects: sometimes the reproductive glands open immediately into its cavity; sometimes the two open externally by a common orifice; and sometimes two distinct orifices, more or less separated from each other, belong to each of the glands. The circulation through this remarkable viscus is of a venous character, and represents a portal system: hence it has been alternately regarded as a respiratory organ, a testicle, and a urinary apparatus; its real office, however, is still problematical.

Gland of Bojanus.

Fig. 254. Gland of Bojanus.

(1334). On throwing injection into the genital orifices*, the sexual glands become tinged, and on examining fragments of such genital glands microscopically, the injected substance may be seen mixed with the ova and spermatozoa. These facts may be observed with special ease in the common Cockle (Cardium edule).

(1335). Sometimes the ova may be seen actually laid by living females of Modiolce and Mytili, one of the valves of whose shell has been removed, on irritation of the genital orifice; and in others, the ova or the spermatic fluid may be made to pass out of their orifices at the breeding season by pressing gently upon the foot. In Spondylus gadevopus, the genital orifice is situated in the sac of Bojanus, where the eggs may occasionally be seen issuing forth, in aspect like a thread of vermicelli, composed of reddish ova mixed with mucus.

* Vide memoir by Dr. H. Lacaze-Duthiers, Ann. des Sci. Nat. 4e ser. t. ii.

(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, without some special provision, the imprisoned animals would, when gravid, be seriously inconvenienced and exposed to continual danger, as the inordinate 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 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.

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

Fig. 255. 1. Oyster (Ostrea 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.

(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 membrane, whilst the two external are distinguished by a structure which merits particular description. Above each external lamina of the gills is a duct proceeding from the posterior part of the foot towards the anal tube, long ago described as an oviduct by Oken, and having, on its lower surface, a long row of openings placed transversely, and forming the entrances to the cells or compartments of the gills themselves. These compartments are all arranged vertically in the gill, and, separated from each other by partitions, they appear as though they originated from the mutual recession of the two membranous surfaces of the gill, which remain connected only by the vertically-disposed vessels that give rise to the septa; they serve for the reception of the ova, which, coming from the ovary placed within the foot, and not by any means in the gill itself, are, however, lodged there, and there receive their further development as in a uterus.