188 SUPPLEMENT 
produced. But if it was exactly this, we ought to find at an 
advanced period of the animal and its shell a part of the im- 
pression which would: be without muscular fibres, and this 
has not been observed, even in oysters and other bivalve 
shells, where it would.be necessary besides that it must exist 
in an inverse direction for every muscle. It therefore may be 
preferable to admit that the muscles grow like all the rest of 
the organization, throughout their whole circumference, but 
especially on the side where the shell grows the most, as 
behind, which is the case with oysters; perhaps it might 
also be supposed that the muscle altogether is at a certain 
point detached when the new stratum of growth is formed, 
and that itis thus that its apparent movement takes place, — 
for the thickness increases equally at the place of the attach- 
ment of the muscles, where the strata are, however, in general 
more crowded, which causes this impression very frequently 
to form a sinking in, and in the fossil state occasions this 
part to be preserved longer than the rest 
The apparatus of locomotion is yet more different in the 
balani'and anatife. In the first the mantle is very slender, 
and presents no muscle but at its posterior or open extremity, 
for the movements of the pieces of the operculum. In the 
second it presents, besides, this singularity, that at its cephalic 
or inferior extremity, in consequence of the position of the 
animal, itis prolonged into a fibro-contractile flexible tube, 
which attaches the animal in a fixed manner to submarine 
bodies. ‘There is, moreover, an adductor muscle between the 
two principal valves of the shell. . 
As to the muscles of the animal itself, or of the trunk and 
its appendages, their disposition evidently begins to approxi- 
mate to that of the entomozoaria. 
The oscabriones have also in the assemblage of their loco- 
motory apparatus something of the true mollusca and some- 
thing of the last mentioned class. In fact, the whole inferior 
a a i 
