224 
CLASS CRUSTACEA. 
Lera, Leach, 
Instead of the stylets at the end of the tail, have but two tu- 
bercles. (Lera ulbifrons, 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, Lair., 
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 byMhe 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 intermediate 
space is raised. The gills are vesicular, imbricated, and 
covered by laminae. 
LlGlA, Fab., 
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. ( Oniscus Oceanicus, Lin.) 
