ON MOLLUSCA. 193 
made by dry muscular fibres or byssus, as some species of 
pectines, the lime, the crenatule, and especially the mussels, 
and the pinne. In this case, it appears that the filaments of 
attachment are fixed to solid bodies, by means of the canalicu- 
late foot with which these animals are provided, and which in 
fact appears very extensible and very long. They cannot 
detach themselves from their position, but if they have been 
thus detached accidentally, they can re-attach themselves to 
their post. 
The arce, and even the tridacne, can also fix themselves to 
solid bodies, by a sort of agglutination of their foot, something 
like the byssiferous species, but en masse, and not fibre by 
fibre. Accordingly, it may happen that from the growth of the 
animal it becomes naturally detached. This is at least pre- 
sumable from the observation that the shell of the tridacne 
loses, in becoming more bulky, the great precordial aperture 
which it had when small, and through which the muscular 
bundle passes. 
In most cases, the acephalous mollusca not being adherent, 
can change place. They move by the assistance of their foot. 
Some, however, are confined to a movement of ascent or 
descent in the hole which they inhabit, whether it be exca- 
vated in a stone, in the sand, or in mud. Their foot, attached 
- more anteriorly than in the other species, comes out more or 
less, is elongated, and takes its point of support on the bottom 
of the lodge. This is the case with all the pyloridz, and per- 
haps a little with the tubicole. 
All the other bivalve mollusca, although again they fre- 
quently live more or less buried in mud or sand, can issue 
from their retreats at will, and consequently completely pos- 
sess the power of motion. Some of them move by leaping, 
almost as if they were impelled forward by a spring. For 
this purpose their foot, very much extended, is bent length- 
wise, and then suddenly straightened. It is this singular 
VOL. XII. O 
