Generative system.—The description of the generative apparatus of the Gasteropoda forms one of the most remarkable parts of their history, and the complication which it presents in some orders is probably unique in the ani mal kingdom. The class may be divided, as far as relates to this function, into three great divisions:— 1st. Hermaphrodite and self-im pregnating; 2d. Hermaphrodite, but recipro cally impregnating each other by mutual copulation ; 3d. Sexes distinct, the female being impregnated by copulation with the male. We shall consider each of these divi sions in the order in which they have been enumerated. The lowest orders approximate the Conchifera in most parts of their organisa tion, and in the arrangement of their generative system we need not be surprised to see a manifest resemblance. The Scutibranchiate and Cyclobranchiate orders, therefore, present this great distinguishing character, which more than any other detaches them from the others, namely, that every individual being furnished both with ovigerous and impregnating organs is sufficient to the impregnation of its own ova. Nothing, in truth, can be more simple than such an arrangement. The ovary is found, when empty, embedded in the substance of the liver, but at certain epochs it becomes so much distended with ova as to cover in great part the rest of the viscera ; from this ovary arises a simple canal or oviduct, which termi nates after a short course in the neighbourhood of the anus. No trace of accessory apparatus has been found, and the only part to which the office of a testis is assignable is the tube through which the ova are discharged, which probably furnishes a secretion subservient to the impregnation of the eggs. Such is the structure of the generative system in Haliotis, Patella, and others of the orders to which these respectively belong, exhibiting a simpli city of parts widely different from what is found in the division which next presents itself to our notice. The second type of the genera tive apparatus Is common to the Nudibran chiatc, lufcrobranchiatc, Tectibranchiate, and Inoperculated pubnonaryorders; in all of which every individual is provided with both male and female organs of copulation, and, accordingly, mutual impregnation is effected by the congress of two individuals, or in a few instances by the combination of several. We shall select the common snail (Helix pomatia) as the most familiar illustration of the general arrangement of the parts composing this double apparatus, leaving the varieties which it presents to sub sequent notice. The admirable plate of Cuvier, of which fig. 190 is a copy, represents the whole system with that clearness and fidelity so characteristic of all the laborious contribu tions to science which we owe to his indefati gable industry. The female portion consists of the ovary, the oviduct, and an enlarged portion of the oviduct which forms a receptacle for the ova, and is called by Cuvier the womb (la matrice). The ovary (q) is a racemose mass embedded in that portion of the liver which is enclosed in the last spire of the body, i. e. that part which is placed nearest to the apex of the shell ; from this proceeds a slender oviduct (r), folded in zigzag curves, and vari ously convoluted : it commences by many small branches derived from the ovary, and terminates in a mass (s), regarded by Cuvier as the testis, in which it becomes so attenuated that it is difficult to trace it ; emerging, how ever, from this mass, it expands into the womb (t), which is a long, capacious, and sacculated canal, and capable of much dis tension, in which the eggs are retained until they have acquired their full development : this viscus opens into the common generative cavity at c, fig. 194.
The male organs consist of a testicle, vas deferens, and penis. The testicle (s,fig.190) appears to be composed of two distinct portions, the larger of which is soft and homogeneous in texture, but the smaller has a granulated ap pearance ; the latter (u) runs along the womb like a mesentery, connecting its folds as far as the termination of that viscus. The testicle
varies much in size at different periods, being generally very small, but during the season of love it dilates so as to fill nearly half of the visceral cavity, at which time the womb like wise is much enlarged. From the testicle arises its vas deferens or excretory duct, which terminates in the penis near the base of that organ. The penis (fig. 194, n) is a most sin gular instrument, resembling a long hollow whip-lash, formed of circular fibres, and, like the tentacles, capable of complete inversion, which in fact occurs whenever it is protruded from the body ; it is also furnished with a re tractor muscle (fig. 190, to), serving to draw it back again after copulation is accomplished. The penis is not perforated at its extremity, but the vas deferens terminates within it by a 1 small aperture, which of course during the inversion of the organ opens externally at about one-third of the length of the penis from its root ; the aperture by which the vas defy rens thus opens upon the exterior of the penis, when that organ Is protruded, Is sufficiently distinct, admitting with facility an ordinary bristle (fig.194, /). On slitting up the penis as it usually lies retracted into the visceral cavity, its inner membrane is found gathered into longitudinal folds, and this provision is needful to allow of that distension which must occur during its erection, at which time this lining membrane becomes the external integu ment of the protruded organ.
These parts would seem sufficient in them selves to fulfil the functions belonging to the organs of both sexes, nevertheless we find others superadded, the uses of which are not so readily assignable; these are the bladder, as it is called by Cuvier, the multifid vesicles, and the sac of the dart.
The sac which has been called the bladder (fig. 190, z, 194, o) is invariably present; It consists of a round vesicle, variable in size, communicating by means of a canal, generally of considerable length and diameter, with the termination of the matrix : it is usually found filled with a thick and viscid brownish matter, and is generally supposed to furnish an enve lope to the eggs as they escape from the con voluted oviduct, an opinion, however, as we shall afterwards see, which is not without op ponents.
The multifid vesicles (fig. 190, x, fig. 194, c) are much less constantly met with, and are in fact almost peculiar to the snail ; they are two groups of caeca, each composed of about thirty blind tubes, which after uniting into larger canals ultimately form a principal duct on each side, through which the secretion which they furnish is poured at a little distance below the orifice leading to the bladder into the passage by which the ova are expelled. The fluid furnished by these curious glandular appendages is white and milky, but as this secretion is almost peculiar to the genus //c/ix, its use is extremely problematic.
The sac if the dart (fig. 190, y, fig. 194, b) is another part of the generative apparatus only found in the snail, and from the extraordinary instrument which it conceals is perhaps the most singular appendage to the generative sys tem met with in any class of animals. It is an oblong sac with strong muscular walls opening by a special aperture into the common generative cavity, like which it is capable of complete inversion. On opening it, its cavity is seen to be quadrangular, and at its bottom projects a four-sided fleshy tubercle, which secretes the curious weapon that this sac is destined to conceal. This (fig. 194, b) con sists of a four-sided calcareous and apparently crystalline spike, about five lines in length, which grows by successive layers deposited at its base from the surface of the fleshy tubercle to which it is attached: it will be evident that when the sac is everted, the dart contained within it will be protruded externally. This dart, if broken off from its place of attachment, is speedily renewed.