230 Description of a Species of Caligus. 
lobe, which is provided, on its interior margin, with a folded mem- 
brane. The lobe is slightly movable upon a joint at its base, and 
the membrane has a very free motion, and serves to close the 
sinus. 
The posterior thoracic segment (F, fig. 7,) is quite short ; 
breadth nearly equals one third of the greatest breadth of pd an- 
terior portion ofthe body. Laterally it terminates in an angle, 
from the posterior side of which, the legs arise which belong to 
this segment. 
The first abdominal segment (G,) differs in form in the two 
sexes. In both, the length and breadth are nearly equal, though 
in general the former is somewhat greater in the female, and the 
latter in the male. The sides are much curved in the male, (fig. 
7,) and the whole is narrower anteriorly. In the female, (fig. 18,) 
the form approaches a square with rounded angles. The poste- 
rior angles.in the male are projecting, and furnished with three 
short hairy sete; the same in the female are provided with the 
same sete, but they scarcely project beyond the adjoining parts. 
These peculiarities only exist in the gravid female. “ When the 
abdomen is destitute of eggs, it resembles that of the male. 
The remaining — J joint, (H, fig. 7,) has a flattened 
subovate form, and is about two thirds the breadth of the prece- 
ding. ‘Two short leaf-like appendages are obliquely articulated 
with its posterior extremity. These leaflets are furnished with 
three terminal plumose sete or pinnule, the cilize of which have 
a length equal to three times the breadth of the seta. 'There are 
two short sets exterior to the pinnule, and one interior. These 
leaflets are ciliated on their internal margin. 
b. Organs appertaining to the several sezments. 
1. Anterior Cephalic Segment.—This segment presents, in its 
front emargination, (A, fig. 1,) two minute rounded papilla, cov- 
ered on their inner surface with very short hairs, which appear to 
correspond to the inner antenne of other crustacea. Below 
just behind their insertion we observe a small semicircular process 
convex outward, which projects a short distance beyond the sut- 
rounding surface. 
Toward the lateral extremity of this segment, on its lower sur- 
face, there is a remarkable organ, which the animal employs in 
= itself, (I, fig. 1 and fig. 19,) but which has heretofore 
been:: considered: its eyes. It consists of a thin nearly circular 
