* The announcement of the discovery of spermatozoa in individuals belonging to these orders, mentioned in a former page, will, perhaps, materially modify the opinions of physiologists upon this point.

(1468). Equally simple is the structure of the generative system in the females of the Pectinibranchiate Gasteropods. A large ovary occupies the same position as the testis of the male, and shares with the liver the interior of the windings of the shell. The oviduct generally follows the same course as the vas deferens of the other sex, and is provided with thick and glandular walls. The eggs, which are very numerous, are arranged in long gelatinous ribands, and, after extrusion, are glued in various ways to the surface of rocks, sea-weed, or even to the shells of other Mollusca. Sometimes, in the siphoniferous tribes, as for example in the common "Whelk (Bitccinum), the ova are enclosed in tough coriaceous capsules secreted by a glandular organ in the vicinity of the oviduct. These capsules contain several eggs apiece, and are joined together in large bunches, such as the waves continually cast up upon every beach.

(1469). The Heteropod Gasteropoda are hermaphrodite. In Ptero-trachea the female organs consist of a distinct ovary, uterus, spermatheca, and an auxiliary gland, all lodged in the visceral sacculus appended to the back. The ovary (fig. 269,p) is of considerable size, and gives origin to a slender oviduct, which, near its termination, communicates with the receptacle for the ova, called the uterus (g.) The spermatheca joins the canal leading from the uterine cavity to the exterior of the body, which likewise receives the secretion of two small glandular sacs (k) apparently destined to furnish some investment to the eggs prior to their expulsion.

(1470). The male parts are situated in the general cavity of the body, quite apart from the female apparatus. The testicles seem to be represented by two wavy caeca (fig. 269, t), which terminate at the root of a small intromittent organ (s) placed at a short distance behind the opening of the vulva.

(1471). All the Tectibranchiata, Inferobranchiata, Nudibranchiata, and the Pulmonated Gasteropods are hermaphrodite, having both a male and female generative apparatus arranged upon the same principles as those of the Snail, which have already been described at length; and to enumerate the variations which occur in the relative position and organization of different parts of the reproductive system in all the genera composing these extensive orders would scarcely answer any useful purpose, even were it practicable within the limits of this work.

(1472). In the male Patella, the testicle is situated upon the right side of the body, between the visceral mass and the external envelope. It is of a pale-yellow colour, with a slight pinkish tint, and seems to be entirely made up of minute tubes, many times folded upon themselves, and imbedded in a granular-looking substance. On cutting into the substance of the testicle, there flows out a milky fluid, which the microscope reveals to contain innumerable spermatozoa, whose movements are very active as long as the seminal secretion is fresh.

(1473). The ovary of the female occupies nearly the same situation as the male testis; but all attempts to trace the excretory duct of either have as yet proved futile.

(1474). When the ova of the Nudibranchiate Mollusca are placed under the microscope soon after the extrusion of the spawn, each is seen to consist of a thin transparent case-membrane*, with a round smooth and opake body in its centre (the ovum proper), which is chiefly composed of minute cells enclosed in a vitelline membrane. These ova vary in size from 1/250th to 1/280th of an inch in diameter. A few hours after the extrusion of the spawn the yelk divides by progressive segmentation until the end of the fifth day, when the division of the cells appears to have reached its utmost limit and the vitelline mass has changed its shape, having become broader at one end, narrower at the other (fig. 276, 2.) At the end of the sixth day no additional change takes place in the external form of the ovum, but the cells into which it has divided continue to coalesce, and minute cilia become apparent on the upper surface of the broad extremity. On the eighth day it assumes the form represented at fig. 276, 3, its circumference becomes more or less translucent, and the external layer of cells seems to separate from the rest to form the commencement of the shell (fig. 276,4), the cilia on the broad extremity become larger and more active in their movements, and traces are observed of the division of this end into ciliated disTcs: it is now entitled to the name of embryo.

Development of the embryo of a Nudibranchiate Mollusk.

Fig. 276. Development of the embryo of a Nudibranchiate Mollusk. 1. Gelatinous coil, in which the ova are imbedded. 2. A portion of the same, magnified. 4,5. Embryos in different stages of growth. 6. Mature embryo when newly hatched, enclosed in a minute nautiloid shell.

* This case-membrane is not the homologue of the ordinary egg-shell, seeing that it sometimes encloses two, three, four, or even five ova.

(1475). From the ninth to the eleventh day the ciliated disks become more developed, more separated from each other, and more moveable; the largest of the four lobes of the body has arranged itself into stomach and intestine, in which occasional contractile movements may be seen.

(1476). The case-membrane previous to the escape of the embryo becomes gradually thinner, and at last either entirely disappears or is reduced to shreds, probably by the incessant strokes upon its inner surface of the long cilia of the ciliated disks during the active revolutions of the embryo round its interior. The embryo at the time of its liberation is provided with a shell somewhat resembling that of the Nautilus, from which it can protrude the anterior part of its body and retract it at pleasure (fig. 276, 6), swimming about actively in the surrounding water by means of its ciliated disks.

(1477). The spawn of other Gasteropods is deposited under diverse forms. In the marine species it is usually found attached to the surface of stones, shells, or sea-weed, the ova being connected with each other in long ribands or delicate festoons, which are sometimes extremely beautiful and curious. The Doris and Tritonia deposit their ova in long gelatinous bands, resembling beautiful frills of rich lace. In Aplysia the spawn resembles long strings of jelly, in which the ova are seen, varying in tint, so as to give different colours to different parts of the thread. In Helix and Bulimus the eggs are protected by a hard shell; whilst in Buccinum, Voluta, Mure.v, and other marine species the ova are enveloped in membranous capsules, agglomerated together in large bunches. These capsules have been sometimes erroneously regarded as the eggs themselves; they are, however, merely coriaceous envelopes, answering the purpose of the gelatinous coating that encloses the eggs of other species. Many of the Gasteropoda are exceedingly prolific: a single Boris will lay 50,000 eggs at a birth; and when we take into account that all the individuals are prolific (the sexes being combined), and that each will produce spawn two or three times in a season, it is evident how vast must be the number of their progeny.