ON CRUSTACEA. 
239 
the symmetry of their tail, by causing it to assume the tur- 
binated figure of the shell. In this case, the terminal appen- 
dages of the tail are transformed into hooks, for the purpose of 
fixing it in its dwelling. That of the other macroura, always 
twice as long as the body, is at first extended in the direction 
of the latter, and bent underneath, at its extremity, which is 
provided with five natatory laminae, simple or double, which 
can be unfolded like a fan, and which, acting simultaneously, 
perform the office of a fin. 
The abdominal or caudal segments are provided on each 
side with small appendages, which have been called false feet, 
and the use of which, in the females, is to serve as points of 
attachment for the eggs. 
The feet of the Crustacea are either proper for walking or 
swimming. Their number, disposition, and more especially 
their functions, differ considerably, for in certain cases some 
of these feet are changed into organs of manducation, and in 
others into respiratory organs. The feet, properly so called, 
are always larger, more solid, and less variable in their forms 
than the others, and especially than those called the branchial 
feet. 
The feet which maybe considered as the normal feet of Crusta- 
cea are constantly formed of six pieces or articulations. Some 
are designated by the name of claws ox forceps, the others are 
called simple feet. Their parts are described in the text, and 
the claws do not differ from the simple feet in their compo- 
sition, but that their penult articulation is more swelled than 
the preceding, is prolonged underneath the last in front, and 
thus forms an immoveable finger ; and that this last articula- 
tion, corresponding in its length to this appendage, is articu- 
lated above, so that it moves upon it from top to bottom, to 
form the forceps. In the brachyuri the forceps are always 
two in number, and belong to the anterior pair of feet, 
except in the genus Pactolus , where it is the last two which 
