. Animal physiology. Physiology, Comparative; Physiology, Comparative. Fig. 12:—Lvmn^us Stagnalts. molluscs, however, the body has the power of forming a shelly covering, which serves for its protection ; but this does not give any assistance in its movements, by affording fixed points for the attachment of the muscles ; in fact, when the animal puts itself in motion, it is obliged to make its locomotive organs project beyond the shell (Fig. 12). Wemustnot regard the shell as an essential *' ?= • part of the Mol- luscous animal; because there are many tribes en- tirely destitute of it; and als
. Animal physiology. Physiology, Comparative; Physiology, Comparative. Fig. 12:—Lvmn^us Stagnalts. molluscs, however, the body has the power of forming a shelly covering, which serves for its protection ; but this does not give any assistance in its movements, by affording fixed points for the attachment of the muscles ; in fact, when the animal puts itself in motion, it is obliged to make its locomotive organs project beyond the shell (Fig. 12). Wemustnot regard the shell as an essential *' ?= • part of the Mol- luscous animal; because there are many tribes en- tirely destitute of it; and also because some of the Articulata have the power of forming a shell, which bears a close resem- blance to that produced by the animals of this group. Not un- frequently we see that, of two animals whose general structure is almost exactly the same—as that of the snail and slug, one possesses a shell, into which it can withdraw its whole body for the sake of pro- tection, whilst the other has none; and that several inter- mediate forms exist, in which the shell bears a larger or smaller proportion to the body, sometimes being able to contain nearly the whole of it, and sometimes being a mere rudiment, as in the Testacella (Fig. 13). 74. The external form of the body of the Mollusca is subject to great variation ; and generally has a good deal to do with the degree in which the organs of sense and the instruments of motion are developed in the particular animal. For these are. Fig. 13.— Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Carpenter, William Benjamin, 1813-1885. London : Wm. S. Orr and Co.
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