ISOPODA. 299 



of at least the four pairs of feet, beginning at the second or third pair, those of 

 the head included. The body, usually filiform or linear, is composed of eight 

 or nine segments, including the head and some small appendages in the form 

 of tubercles at its posterior and inferior extremity. The feet are terminated 

 by a stout hook. The four anterior, the second of which are the largest, are 

 always terminated by a monodactyle forceps or a claw. In several, the four fol- 

 lowing ones are shortened, less articulated, without the terminal hook, or are 

 rudimental, and nowise adapted for the ordinary uses of similar parts. 



The females carry their ova under the second and third segments of the 

 body, in a pouch formed of approximated scales. They are ah 1 marine Crus- 

 tacea. 



We may unite them in a single genus which, by the law of priority, should 

 be called 



CYAMUS, Latreille. 

 Now divided into Leptomera, Naupredia, Caprella, and Cyamus proper. 



ORDER V. 



ISOPODA*. 



THE Isopoda approach the Lsemodipoda by the absence of the palpi of the man- 

 dibles, but are removed from them in several other respects. The 

 two anterior feet are not attached to the head, and belong, as well as 

 the following ones, to a particular segment. They are always 

 fourteen in number, unguiculated, and without any vesicular ap- 

 pendage at their base. The under part of the tail is furnished 

 with very apparent appendages resembling leaflets or vesicular 

 burssE, the two first or external of which, either partially or wholly, 

 usually cover the others. The body is generally flattened, or is wider than it is 

 thick. The mouth consists of the same pieces as in the preceding Crustacea ; 

 but here, those which correspond to the two superior foot-jaws of the Decapoda, 

 exhibit an appearance of a lower lip, terminated by two palpi, still more than in 

 the latter. The two mediate antennae are almost obliterated in the last Crusta- 

 cea of this order, which are all terrestrial, and also differ from the others in their 

 respiratory apparatus. Most of them are aquatic. Those which are terrestrial, 

 like all other Crustacea which live out of water, still require a certain degree 

 of atmospheric humidity to enable them to breathe, and to preserve their bran- 

 chiae in a proper state for the exercise of that function. 



* Equal-footed. 



