The otocysts are always paired, and, in Gasteropoda, are placed close to the pedal ganglia. The acoustic nerve, however, has been shown by Lacaze-Duthiers to connect with the cerebral ganglia in certain cases. The otocysts are never on the surface of the body and are rarely connected with it by any passage or tube; it is probable therefore that sound reaches them simply through the medium of the tissues.
In the Pelecypoda the otocyst is similarly situated near the pedal ganglion, and is probably (though this has not yet been proved) similarly connected with the cerebral. There is only a single otolith. Pelseneer finds[308] in Nuculidae alone a free communication between the otocyst and the exterior. Anodonta has been observed[309] to withdraw its foot into the shell at the noise of an opening door, a loud voice, or a shrill whistle, whether in a basin of water or lying on a study table.
Fig. 96.—Illustrating the otocyst in A, Anodonta, B, Cyclas; ot, otolith; a, b, c, c´, cellular layers surrounding the chamber; ci, cilia on interior walls of chamber: C, an otolith crushed. (After Simroth.)
Delage extirpated the otocysts in certain Octopoda, and obtained some unexpected results. He found that remarkable effects were produced upon the animal’s powers of locomotion, so that it was unable to preserve its proper balance in the water when in rapid motion, but its body was forced to undergo a form of rotation more or less pronounced. He concluded that the otocysts must possess, besides their auditory functions, a power which stands in some relation to the proper orientation of the body in locomotion, a power which is not wholly supplied by sight and touch alone. The otocysts may thus regulate locomotion by stimulating muscular acts which tend to keep the body in the straight line during the process of movement.[310]
The Foot
One of the most characteristic organs of the Mollusca is the foot, which, under one form or another, occurs throughout the whole phylum. The foot is a thickening, on the ventral side, of a portion of the integument of the animal, modified to serve different forms of motion. It attains its maximum relative area in the Chitonidae, many Nudibranchs, and the slugs generally, in nearly all of which there is no portion of the body which is not subtended by the foot. Here too it presents the form of a regular disc or ellipse, which is more or less produced. In many cases, however, the foot becomes modified in such a way that we are enabled to recognise well-marked anterior and posterior portions, which have received the name of propodium and metapodium respectively, while the intervening central portion is termed the mesopodium.
Fig. 97.—Sigaretus laevigatus Lam., showing excessive development of the propodium (pr) and metapodium (met) in a mollusc living in sand (the shell, which covers only the liver and adjacent parts, has been removed); l, liver; s.ap, aperture of proboscis, here deflected from the median line; t, t, tentacles. (After Quoy and Gaimard.)
The propodium is most strongly developed in genera which crawl about in wet sand, e.g. Natica, Sigaretus, Oliva, Harpa, Scaphander (Figs. [97] and [98], and compare Fig. [91]). In such cases it seems to serve as a sort of fender or snow-plough, to push the sand away on both sides of the path the animal is traversing. In some species of Sigaretus the propodium becomes as it were banked up against the head and proboscis, which are thus unnaturally elevated, or tend to disappear altogether. Bullia (Fig. [62]), which crawls about rapidly on wet sand, appears to attain its object by a wide extension of the foot on all sides, and so slides over the sand instead of ploughing through it; the little lappets at the end of the ‘tail’ probably serve as rudders.