Anatomy of Buccinum.

Fig. 275. Anatomy of Buccinum.

(1461). The tentacula (fig. 275,ff) in the marine Gasteropoda are generally not retractile, and the eyes are frequently situated at the outer side of the base of each tentacle, instead of at their apex as in the figure referred to; but, with these exceptions, we can add nothing to what has been said concerning the senses of these Mollusca in the description of the Snail already given as an example of the general structure of the entire class.

(1462). The organ of hearing is now universally admitted to exist in all the Gasteropod Mollusca, and, according to Siebold*, is invariably situated in the immediate vicinity of the two most voluminous cerebral masses. Like the other organs of sense, the organ is always double. It is formed 1 by two hyaline ovate or orbicular capsules, situated on the head or neck at the bases of the tentacula, and is supplied with its specifically-endowed nerve from the cerebral ganglions. In the capsule there are enclosed one or several (and sometimes they are numerous) oval or round crystalline bodies, named otoliths; and it is observable that the number varies not only in neighbouring genera, but even in nearly-allied species. Siebold says that a concentric depression is evident in these otoliths, and there may be seen, in the greater number of them, a shaded spot, or, rather, a minute aperture, which penetrates through the concretion from the one flattened surface to the other. Subjected to a strong pressure, the otoliths crack in radiating lines, separating often into four pyramidal pieces.

This separation also ensues when the otoliths are immersed in diluted nitric acid; and if we touch them with the concentrated acid, they suddenly dissolve, with the disengagement of a gas, whence Siebold concludes them to be composed of carbonate of lime. The size of the otoliths is not equal; and in the same capsule there are always some which are smaller than others. "Within the capsule they have, during life, a very remarkable, and in some respects peculiar, lively oscillatory movement, being driven about as particles of any light insoluble powder might be in boiling water. The otoliths in the centre have the appearance of being pressed together, so as to form a sort of solid nucleus; and towards this centre the otoliths seem ever to be violently urged, their centripetal rush being invariably repulsed, and themselves driven back again in a centrifugal direction. Removed from the capsule, the motions of the otoliths instantly cease. The cause of these curious oscillations remains undiscovered. Siebold could detect no vibratile cilia on the surfaces of the capsule 2; and the cessation of the motion when the otoliths are removed proves them to be unciliated themselves, and at the same time distinguishes the motion from that of inorganic molecules, as described by Mr. Brown.

(1463). Dr. Nordmann, in an elaborate memoir on the anatomy of the Tergipes Edwardsii, minutely describes the structure of the auditory capsules of that species, in which they are found situated immediately behind the eyes, upon the posterior portion of the two anterior ganglia, and are at once recognizable by their sharp outline and very considerable size, which surpasses that of the eyes themselves. The proper auditory nerves are wanting, the vesicles of hearing being lodged in little excavations in the ganglia themselves. These vesicles, which are of a round or oval shape, consist of a thin vitreous-looking membrane (but which is sufficiently tough to resist considerable pressure), and contain a fluid in which is suspended a minute rounded otolith.

* Ann. des Sci. Nat. 1843, xix. p. 198.

1 Introduction to Conchology; or, Elements of the Natural History of Molluscous Animals, by George Johnston, M.D., LL.D., to which the student is referred, as being by far the best treatise upon the subject in the English language, for fuller details concerning the habits and organization of the Mollusca.

2 Kolliker has observed that the motion of the otoliths in the Mollusca is dependent upon cilia, with which the internal surface of the auditory cyst is covered.

(1464). We now approach an inquiry of much interest as concerns the economy of the animals before us - namely the varied forms of their organs of reproduction, and the character of the generative system belonging to each order. This investigation, however, is one of no ordinary difficulty; for so numerous are the modifications of structure observable in almost every genus, that, were we not strictly to confine ourselves to the study of the most prominent and important features of this portion of their history, the patience of the student would be severely put to the test in following us through all the details connected with so extensive a subject.

(1465). The three lowest orders of the Gasteropoda are still, in many particulars, more or less allied to the Conchipera; but more especially this is the case in the organization of their generative apparatus. The Cyclobranchiata, Scutibranchiata, and Tubulibranchiata, like the inhabitants of bivalve shells, are all hermaphrodite and self-impregnating *. A large granular ovary is in all these orders imbedded in the mass of the liver; and from this a duct leads to an external orifice situated in the vicinity of the anus: if impregnation is in such animals essential to fecundity, the fertilizing secretion must be furnished by the glandular walls of the oviduct, as no male organs have as yet been discovered.

(1466). The Pectinibranchiata, on the contrary, are all dioecious, - the sexes being distinct, and intercourse between the male and female necessary for the impregnation of the latter.

(1467). The male is generally at once distinguished by the penis, appended to the right side of the neck (fig. 275, g), an organ which is frequently of enormous proportions - so large, indeed, that, it being impossible that it should be retracted into the body, it is generally simply folded back into the branchial chamber. The testicle is imbedded in the mass of the liver, and lodged in the inmost recesses of the shell. It gives origin to a long and very tortuous vas deferens, which is at first extremely slender, but on emerging from the mass of the viscera becomes thicker, running along the right side of the body until it enters the penis, and, having made many zigzag folds, reaches the extremity of that organ, where it terminates by a small orifice.