932 SUPPLEMENT 
which are but comparatively few, do not, however, exhibit 
many differences in this respect, although some remain fixed 
to the surface of the ground, like oysters ; such are the etheriz, 
according to the discovery of M. Caillaud; others adhere to 
submerged bodies by means of a byssus, as the mussel of the 
Danube ; others move about in the mud and on its surface, as 
the unio and anodontes ; and finally, others live much deeper, 
and also move, as the cyclades. 
The circumstances of the life of the marine mollusca are 
much more variable. Thus the majority live on the sea coasts, 
on rocks, and in places in the eddies and mouths of rivers, 
which are hence called littoral species. But there is a cer- 
tain number of others which appear to exist only at distances 
more or less considerable from the shore, and at great depths, 
which causes them to be distinguished under the name of pela- 
gian mollusca. The terebratule appear to be in this case, 
and it is supposed that the nautili and ammonites are still 
more so. In fact, the calamary, the sepie, and the spirule, to 
which they approximate, are animals of the high seas. 
We next find, that according to their mode of locomotion, 
some mollusca live continually swimming or floating at the 
surface, or in the interior of the waters, or creeping on the 
rocks in the middle of the varecs which cover them; others 
are attached there in a fixed manner by their shell, or by a 
byssus, or, finally, sunk more or less deeply into sponges, in 
the mud, in the sand, in rocks, in madrepores, in other shells, 
and even in non-calcareous stones, as well as in dead or living 
wood. 
The species which live in sponges, are in the same case as 
mussels, &c. which are found in the holes of rocks; but as the 
substance, in the excavation of which they have been acci- 
dentally placed, increases, as long as it is living, the result is, 
that they are finally completely enveloped in it, to that degree, 
no doubt, that they are as it were suffocated. 
