We have hitherto abstained entirely from mixing up with our description of these prin cipal forms which the generative system of the mutually impregnating Gasteropoda presents, the discussions which have arisen concerning the real nature of the different organs which have been described, and have designated them by the terms usually applied to the respective parts,without reference to their individual func tions. It now, however, becomes necessary to lay before our readers the principal opinions which are recorded upon this subject. The chief points of debate have been the bladder, and the organ which we have described under the appellation of testicle. The bladder is, from its constant occurrence, evidently an organ of some essential use : it was regarded by Swammerdain as the secreting structure from which the colouring fluid peculiar to some species is produced, especially in the Murices and others of the marine genera ; it was there fore named by him sac of the purple ; but we shall afterwards find that this fluid is derived from another source. 131ainvillc, on the other hand, considers this vesicle as analogous to the urinary bladder of Vertebrata ; in reference to this hypothesis, however, we should be inclined to ask, with Cuvier, where are the kidneys? and even upon the supposition that the secretion of the bladder was analogous to the urinary fluid, we are not aware of any chemical proofs of its nature which are sufficient to establish the identity. Belle Chiaje again sustains that the sac of the purple is, in fact, the testis, and that its secretion, poured as it constantly is into the termination of the oviduct, is in re ality the fecundating fluid ; yet against this we must urge the distribution of the vas deferens met with in the Helices, which from its entire arrangement converts the organ of excitement in these animals into an apparatus of immis sion, whose nature cannot be mistaken. The opinion which we consider most consonant with all the circumstances of its position, is that it is a reservoir for the seminal fluid analogous to the spermotheca of certain insects. Cuvier expressly notices the constant relation which exists between the length of the penis and that of the canal which leads to this sac culus, and when we remark the long chains of ova which are slowly extruded in most of the Gastcropoda, we are readily disposed to admit of the necessity of such a reservoir, which, treasuring up the semen until the eggs are about to be expelled, applies it efficiently to the ova as they successively pass the orifice of its duct. This supposition derives additional weight from what we have found to be the arrangement of the seminal ducts in Vaginulus and Onrhidium. In the former we observed that, besides the canal, which, as in the Snail, perforates the root of the penis and thus be comes subservient to copulation, the vas de ferens actually pours a part of its contents by a separate canal into the bladder itself, which, as in all cases, communicates with the egg passage. In Onchidium the connexion be tween the testis and this receptacle is equally striking, as will be obvious on reference to the drawing given above. In Aplysia, Delle Chiaje considers the testicle as described by Cuvier to be in reality the matrix or receptacle for the ova, in which they attain their full development prior to expulsion, basing his opinion upon the disposition of the spiral cavity which it contains.
We are entirely left to conjecture as to the uses of the other appendages found in par ticular species, and the multifid vesicles of the Snail, which are wanting even in the Slug, the tortuous canal connected with the penis of Doridium, and the still more singular organs belonging to the male apparatus of Onehidium, must still remain the subjects of observation and experiment.
The third form of the generative system in which the sexes are distinct, is met with in all the Pectinibranchiate order, and in the operculated Pulmonalia of Ferussac. In Buc cinum, which we shall select as an example of the general arrangement of the sexual organs in the former, the male is at once distinguish able by the enormous penis attached to the right side of the neck (fig. 190), which is not, as in the last division, capable of retraction within the body, but remains permanently ex ternal, being, when not in use, folded back and lodged within the branchial cavity, from which however it is frequently protruded without any apparent object.
In the female there is no rudiment of such a structure, but the generative aperture is seen to be situated a little within the edge of the pulmonary cavity, being a simple hole leading to the oviduct. The internal organs of the male, represented in the annexed figure, con sist simply of a testicle and its excretory canal. The testis is of considerable size, sharing with the liver the smaller convolutions of the shell; from this arises the as deferens, which forms by its convolutions a kind of epididymis (fig.199, b), and then increasing in diameter enters the root of the penis, through which it passes by a tortuous course (d) to the tubercle at the extremity of this organ, where it opens externally. The penis when opened, as re presented in the engraving, is seen to contain strong transverse fhseiculi of muscle, which probably cause the erection of this organ ; they will at the same time lengthen it, so as to destroy in a great measure the zig-zag turns into which the vas deferens is thrown in its usual relaxed state.
In the female the position of the testicle is occupied by the ovary, while the vas deferens is represented by a thick and glandular oviduct.
In the penis of the male is pro portionally smaller ; and, instead of a com plete vas deferens, penetrating to its extremity, there is merely a groove along its surface, along which the semen flows. In Volula the ex terior groove only runs to the base of the penis, and in Strombus the male organ is a mere tubercle.
In the Pubnonalia operculala the organs of both sexes are in every respect similar to those of the Pectinibranchiate order. In Paludina alone ( Helix vivipara, Lin.) the penis is retrac tile,issuing from ahole found in the right tentacle, and from the disparity in size between the tentacles, arising from this cause, the male is readily distinguished. The females of this genus are not unfrequently ovo-viviparous, the ova remaining in their capacious oviduct until they are hatched.
Spallanzani asserts that, if the young of Paludina are taken at the moment of their birth, and kept entirely separate from others of their species, they can reproduce without im pregnation, like the Aphides and Monoculi, in which the same connexion with the male is found to fecundate not only the female herself, but her offspring for several generations. Nevertheless, whether Spallanzani s observa tions be correct or not, the males are fully as numerous as the females, so that it wot.ld be difficult to imagine the object of such a de viation from the ordinary proceedings of na ture.
Ova.—The spawn of the Gasteropod Mol lusca is found under diverse forms ; it is usually in the marine species attached to the surface of stones, shells, or sea-weed, the ova connected with each other in long ri band's or delicate festoons, which are some times extremely beautiful and curious. The Doris and Tritonia deposit their ova in this manner, and the mass of eggs deposited by them resembles a frill of lace of extreme beauty. In Aplysia the spawn is found to resemble long gelatinous threads, in the centre of which the ova are seen, varying io tint, so as to give different colours to different parts of the thread ; the whole strikingly resembles strings of vermicelli, and the Italians in fact have applied to them the name of vermicelli marini. In Helix and Bulimits the eggs are naked and protected by a hard shell, whilst in Buceinum Voluta, Murex, and other marine species, the ova are enveloped in membranous sacs agglomerated together in large bunches; these sacs have been erroneously regarded as the eggs themselves; they are, however, merely coriaceous envelopes, answering the purpose of the gelatinous coating enclosing the eggs of other species, several eggs being contained in each bag, in which, when mature, the young are easily seen. It would seem that extraordinary provisions have been made by nature for the multiplication of these creatures, in spite of the numerous enemies which devour them, or the vicissitudes of temperature to which, espe cially in the terrestrial species, their eggs are necessarily subject. We are indebted to Al. Leuchs for several interesting observations concerning the ova of slugs, which explain in a great degree the quantities of them which in some seasons infest gardens and vineyards, becoming, froth the devastation which they cause, serious plagues to the agriculturist. The number of eggs varies with the healthiness of the animal, the supply of food, or the tem perature of the season ; yet it is probable that a single slug will lay five hundred, under ordinary circumstances: thus, supposing a thousand of these creatures to be collected in a given space, they will give birth in a few weeks to five hundred thousand young slugs, which multiplying in their turn would pro duce at the second laying two hundred and fifty millions of eggs. This fact is well worth the notice of the farmer, who, instead of dri ving away with so much assiduity crows and other birds which live upon these destructive, though apparently insignificant, animals, would do well occasionally to cherish them as fellow labourers in his grounds. The Terrestrial Mol lusca, helpless and incapable of defence, afford food to numberless indefatigable assail ants, and their preservation is provided for, not only by the number of their eggs, but by a peculiar tenacity of vitality which these ex hibit under circumstances which would be thought sufficient to destroy the young before they were hatched. The skin of the eggs of the slug is coriaccous and very elastic, so that when compressed they soon resume their shape : exposure to intense cold does not de stroy their fertility, and they have been known to resist a temperature of 400 without ap parent injury. When dried by artificial heat, they shrivel up to minute points only distin guishable by the microscope, yet in this state, if they be put into water, they readily absorb it and are restored to their former plumpness. The same thing happens to those which are dried by the action of the sun and apparently destroyed; a shower of rain is sufficient to supply them with the fluid which they had lost and to restore their fertility. This drying ap pears not to injure them. M. Leuelis found that after being eight times treated in this manner, they were hatched on being placed in favourable circumstances, and even eggs in which the embryo was distinctly formed, sur vived such treatment without damage.