184 _ SUPPLEMENT 
no retractor muscles, we do not the less find the retractor 
muscles of the tentacula. But their origin takes place above 
the muscular partition, which separates the visceral cavity 
from the pulmonary cavity, pretty nearly at the same point as 
in the conchyliferous species. 
It is also nearly from the same point that proceeds the re- 
tractor muscle of the excitator organ when it-exists. 
Finally, it is also from the muscular: bundle of the colu- 
mella that springs the retractor muscle of the operculum, 
where this part exists, and to which is attached the siphon of 
such species as are provided. with one. 
We have observed, a little farther back, that some cephalous 
mollusca are provided on each side with locomotory ap- 
pendages, pretty considerable, as the sepiz, calmars, and 
pteropods generally speaking. In this case these appendages 
have levator or depressor muscles, which are carried from the 
back or the belly to their roots. But when the appendages 
do not really serve for locomotion, they are formed of a con- 
tractile dermis, in which it is not possible to distinguish true 
muscles. 
The acephalous mollusca present a disposition of loco- 
motive organs considerably different, and which appears still 
more so when the passage from the cephala to the acephala 
has not been very precisely observed, as in all the mollusca; 
in general, all the parts of their envelope, whether branchial 
or not, are really contractile. But we sometimes remark 
besides some distinct muscular fibres, which, from the 
edges of the mantle, more or less thick, proceed to fix them- 
selves to the shell at alittle distance from its circumference, so 
as to be capable of drawing those edges in more or less; and 
more rarely we perceive small slender muscles, which pro-: 
ceeding from the adductor muscles, of which we are about to 
_speak, direct themselves into the different points of each lobe 
of the mantle. In the cases where the latter has only this 
