In many of the Tectibranchiata a remark able arrangement of the generative organs is found, as the male viscera are divided into two distinct portions, the exciting organ being at one extremity of the body, while the testis is found connected with the female apparatus in a distant part of the system. This will be seen in Doridium Illeckelii (fig. 196); the penis (/), seen retracted in the figure, issues from the side of the neck, and has appended to its root a zig-zag tube, inclosed in a mem branous canal, the nature of which is un known. Quite detached from these, and placed near the anus, we have the matrix (f), the testis (g), and the bladder (i), occupying their usual relative position as regards each other, and terminating in the vulva or sac of generation (h).
In Aplysia the organ of excitement is found near the right tentacle, where it protrudes, as in the Snail, for the purpose of copulation, by the inversion of its walls; it is, however, absolutely imperforate, and receives no duct by which it can communicate with the testis so as to become instrumental in immission; but externally a deep groove is seen upon its surface when in a state of protrusion, which is continuous with a long furrow seen upon the surface of the body, continued from the base of the penis to the orifice of the female ap paratus. Fig. 197 represents the secreting portions of this system removed from the body, and displayed so as to expose the internal structure of the parts composing it. The ovary (h) is a large oval, whitish, and granular mass, from which the oviduct arises by several distinct tubes which emerge from different parts of its substance : this oviduct opens into the common tube (e), which may be called the vagina. The mass (f, g), called by Cuvier the testis, and supposed by him to be solid and homogeneous in its texture, is found, when opened, to be divided by spiral septa, resem bling the scala cochleae in the ears of 111am malia (g), and thus forms a long spiral cavity communicating with the commencement of the vagina, in which latter tube we also find aper tures by which the vesicle (p) and the larger sacculus (o) communicate with the common passage.
Onchidium, an aquatic species belonging to the inoperculate pulmonary order, the male and female parts are in a similar manner placed at opposite extremities of the body, but the former assume a more complicated structure than in the Tectibranchiata, which we have described. The ovary (fig.198, A, a, a) consists of two masses replete with ova, each of which furnishes a short duct ; the two thus formed unite into a convoluted tube (b), which is the common oviduct: arriving at the mass always regarded by Cuvier as the testis, it enlarges and forms within the substance of that organ many convolutions, on emerging from which it runs directly in the shape of a narrow canal (d), to the external orifice (h).
The bladder (f) receives a large duct (e) from the mass here assumed to be the testis, and gives off another of equal size, which joins the oviduct (d) prior to its termination. This would seem to form a complete system in Itself; yet, on examining the male organ of excitement, we find it connected with considerable appendages, the nature of which it is difficult to conjecture. The sac (in) is muscular, and resembles the mus cular root of the penis in the genera already described, being, as in them, capable of in version : at its base are seen two cul-de-sacs, into each of which opens a long and flexuous canal (1, a). The canal marked n is very slender, and when unfolded is four times the length of the body of the animal ; its termi nation at the point most remote from the mus cular sac into which it opens is apparently closed. The other tube marked 1 is much wider and of extraordinary length ; its com mencement (i) is extremely convoluted and fully eight times as long as the body ; its walls are thin, but it is supplied plentifully with blood by means of a large artery interlaced with its convolutions ; at k it becomes enveloped in a fleshy mass of considerable thickness, after which, assuming its original appearance, it proceeds to the cul-de-sac, at the bottom of which it terminates. In fig. a, 198, the muscular cavity (m) has been laid open, and the mode in which the above tubes enter it has been displayed ; the smaller one (n) ends in a little horny papilla (q) seen in the engraving ; the larger tube (1) terminates by a kind of glans penis, perforated by a large aperture and sur rounded by a kind of prepuce (p): on open ing the vessel a little before its entrance into the muscular sac, it is found to conceal a sharp horny dart (o), supported upon a fleshy pedicle, and readily protrusible through the aperture p; the analogy between this singular instrument and the dart of the Snail is ob vious, for when the muscular sac (m) is everted, the papillae (p, q) become external, and the horny point being pushed out of the former will probably form a stimulus of the same description.