Page:Natural History, Mollusca.djvu/115

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103
NAKED-GILLED MOLLUSKS.

lively action, sinking, or rising, or advancing onwards at its pleasure."

Messrs. Alder and Hancock, from whose beautiful "Monograph of the British Nudibranchiate Mollusca" I cite these particulars, "have not succeeded in tracing the development of the larva into the mature form; but it is not difficult to understand how this change is effected. When the larva is placed with the mouth of the shell downwards, the oral lobes in front, (speaking particularly of Eolis,) the anal termination of the intestine, and the oval sac representing the generative organs, will be found on the right side, close to the base of the oral lobe, and the operculigerous lobe or foot will be seen to extend backwards in a median position, occupying the place of the crawling disk. Thus it is evident that the principal organs of the larva only require to be slightly modified in form, and it is changed into the mature animal, the shell and operculum being cast off, and the oral lobes either absorbed or altered into a veil or oral tentacles."

The food of the Nudibranch Mollusca is for the most part animal. Various kinds of zoophytes are devoured by many species, some are even cannibals, preying upon their own kind; but some of the Dorides appear certainly to be herbivorous, feeding on the fronds of sea-weeds. Professor Grant has often found their stomachs completely filled with minutely divided portions of coarse marine plants, filling not only that organ, but also the cavities of the liver, which in these animals is very large. The mouth and its various parts are very efficiently adapted for the seizing and devouring of food. Various parts are armed with points and cutting