22i CLASS CRUSTACEA. 



J^RA, Leach, 



Instead of the stylets at the end of the tail, have but two tu- 

 bercles. {Jcera iilhifrons, Leach.) 



Finally, the isopods of the sixth and last section, Onis- 

 ciDES, Lat., have four antennae, but the two intermediate 

 very small, but little apparent, and of two articulations at 

 most. The lateral are setaceous. The tail is composed of 

 six segments, with two or four appendages, in the form of 

 stylets, at the posterior edge of the last, and without lateral 

 fins. Some are aquatic and others terrestrial. In the latter 

 the first leaflets of the end of the tail present a range of little 

 holes, through which the air penetrates, and is carried to the 

 organs of respiration, which are inclosed there. 



Some have the sixth articulation of their antennae or their 

 stem composed in such a manner, that, counting the little arti- 

 culations of this part, the sum total of all the articulations is 

 nine at least. These isopods are marine, and form two sub- 

 genera. 



Tylos, Latr., 



Appear to have the faculty of rolling themselves into a ball. 

 The last segment of the body is semicircular, and fills exactly 

 the notch formed by the preceding. The posterior appen- 

 dages are very small, and entirely inferior. The antennae 

 have but nine articulations, of which the last four compose 

 the stem. On each side is a sunken tubercle, representing 

 each, one of the intermediate antennae. The interaiediate 

 space is raised. The gills are vesicular, imbricated, and 

 covered by laminae. 



LiGiA, Fah., 



Have the stem of the lateral antennae composed of a great 

 number of small articulations, and two very projecting stylets, 

 divided at the end into two branches, at the posterior extre- 

 mity of the body. [Onisciis Oceanicus, Lin.) 



