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 Jeet, 

 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 or 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 



