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_. [Illustration: 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.