NAKED-GILLED MOLLUSKS. 103 
lively action, sinking, or rising, or advancing on- 
wards at its pleasure.” 
Messrs. Alder and Haneock, from whose beau- 
tiful “Monograph of the British Nudibranchiate 
Mollusca” I cite these particulars, “‘ have not suc- 
ceeded in tracing the development of the larva into 
the mature form; but it is not difficult to under- 
_stand how this change is effected. When the 
larva is placed with the mouth of the shell down- 
wards, the oral lobes in front, (speaking particularly 
of Holis,) 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 ten- 
tacles.”’ 
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 
