204 SUPPLEMENT 
Finally, for the species whose nutriment consists of mole- 
cules already disunited, or of microscopical bodies suspended 
in the fluids in which they live, there is no necessity of re- 
search or prehension. It is sufficient for the animal to pro- 
duce in the water an almost circulatory movement of this fluid 
which should bring with it the nutritive substance, and pro- 
bably to swallow this substance and the vehicle at once. 
The mode in which the species with a spiral tongue use 
that organ is totally unknown. 
We know but little more concerning the method by which 
the acephala take their food. It would appear that it must be 
almost in a molecular state, suspended in the water, and carried 
by the buccal appendages to the mouth ; for there is no indi- 
cation of masticatory and salivary apparatus. 
The palliobranchia, by the aid of their long labial append- 
ages, should be better able to seize their food, since they can 
put them forth out of their shell, and agitate them in all direc- 
tions. The ascidiz and biphore, having no appearance of 
such apparatus to the mouth, must be placed in a situation 
altogether opposite. 
Deglutition, at least in the cephala, must take place as in 
animals of a higher grade. 
The organs of respiration are pretty nearly known in all the 
true malacozoaria and in all the malentozoaria ; but they vary 
considerably, not only with respect to form and situation, but 
even as regards structure. 
In fact, in this last respect, although in most mollusca the 
part of the exterior envelope, modified to form the organ of 
respiration, be disposed like gills—that is, in such a manner 
that it shall be an organ capable of immersion in the ambient 
fluid—it sometimes happens that there is a contrary disposition, 
and that it forms a sort of pouch or cavity, into which the 
ambient fluid shall penetrate, which constitutes an aérial or pul- 
monary organ ; and then the afferential and efferential vessels 
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