to complete our description of the parts composing this complex organisation, it remains only to mention the common generative cavity 194, a), into which the others open; this, when in its ordinary position, is a muscular bag, opening externally by a large aperture near the upper tentacle on the right side of the neck, whilst at its bottom are seen the orifices of three distinct passages, one leading to the penis, one to the female organs, and a third to the sac of the dart. This cavity, like that of the dart, is capable of inversion, which is effected partly by the action of its muscular walls, aided in all probability by a kind of temporary erection, and when thus turned in side out, the orifices leading to the penis, tlio womb, and the sac of the dart of course become external.
In order to understand the functions of these various parts it will be necessary to describe at length the singular mode in which copulation is effected. When two snails, amorously dis posed, approach each other, they begin their blandishments by rubbing the surfaces of their bodies together, touching successively every part. This preliminary testimony of affection lasts for several hours, gradually exciting the animals to more effective demonstrations. At the end of this time the generative orifice, placed on the right side of the neck, is seen to dilate, and the common generative cavity becoming gradually inverted displays ex ternally the three apertures which open into it. This being effected, an encounter of a truly unique character commences; the opening leading to the sac of the dart next expands, and that organ undergoing a similar inversion displays the dart affixed to its bottom. A series of manoeuvres may then be witnessed of an unaccountable description ; each snail, in turn, inspired with an alacrity perfectly foreign to its ordinary sluggish movements, striving with his dart to prick the body of his associate, which with equal promptitude endeavours to avoid the wound, retreating into his shell, and performing a variety of evolutions to get out of reach. At length, however, the assailant suc ceeds, and strikes the point of his weapon into the skin of his paramour at any vulnerable point which may be found. The dart is gene rally broken off by this encounter, sometimes sticking in the skin, but more frequently dropping to the ground. The reptile Cupid having thus exhausted his quiver, becomes in turn the object of a similar attack, exhibiting apparently an equal anxiety to avoid the threat ening point of the weapon bared against him. At last he receives the love-inspiring wound, and the preliminaries thus completed, each prepares for the completion of their embraces. The other two apertures next dilate, and from one of them issues the long and whip-like penis, unrolling itself like the finger of a glove ; this being fully developed is introduced into the vaginal orifice of the other snail, which in the same manner inserting its penis into the female aperture of the former, both mutually impreg nate and are impregnated. See fig. 195.
It is difficult to conceive what can be the use of the dart so singularly employed; it would seem to be an instrument for stimulating the sleeping energies of the creatures to a needful pitch of excitement; yet why it should be peculiar to the snail is not obvious, for in the slug and other AIollusca certainly not less apathetic, no such structure has been detected.
In Vaginulus (fig. 189) a similar arrange ment of the principal organs is observable, although some modifications are met with which deserve our notice. No sac of the dart is found in this animal, but a fasciculus of caeca, analogous to the multifid vesicles as far as their structure is concerned, is connected, not with the female apparatus, as in the snail, but with the male organs. The orifices of the two sexual systems are here separated by a considerable interval, the penis emerging at the side of the neck, near the right superior tentacle at z, while the orifice of the female parts is placed between the cuirass and the mantle, considerably further back. The ovary (in) is similar in structure to that of the snail ; and its duct, in like manner, forms many convolutions in the substance of the testicle (p), from which it issues, much increased in size, to expand into a large membranous re ceptacle (q), corresponding in function with the tortuous matrix of the Helices; this part of the oviduct is filled with an albuminous fluid, and from it runs the narrower canal (r), which may be regarded as the vagina, and which before its termination communicates with a lateral pouch, identical with what has been called the bladder. The testicle (p) ap pears to consist of two portions, from which arises the vas deferens (a). On tracing this tube it is seen to divide into two branches, one opening into the bladder (s), an arrangement to which we shall again have occasion to revert, whilst the other runs forward to the root of the penis (Sc). The latter organ presents two por tions, a long tubular crecum (v), resembling the corresponding part in the snail, and a thick muscular cavity, from which the former arises as a kind of appendage ; on opening the thicker portion its interior is seen to be rugose, and to enclose a small body, something like the caput gallinaginis in the human urethra. The nrultifid vesicles (y) open near the exterior orifice, through which the whole apparatus, by a process of inversion already described, is protruded so as to form the male organ of ex citement.