230 Description of a Species of Caligiis. 



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; its 

 breadth nearly equals one third of the greatest breadth of the an- 

 terior portion of the body. Laterally it terminates in an angle, 

 from the posterior side of which, the legs arise which belong to 

 this segment. 



The frst 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 setae ; the same in the female are provided with the 

 same setas, but they scarcely project beyond the adjoining parts. 

 These peculiarities only exist in the gravid femala.^ When the 

 abdomen is destitute of eggs, it resembles that of the male. 



The remaining abdominal 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 setas or pinnule, the cilise of which have 

 a length equal to three times the breadth of the seta. There are 

 two short setce exterior to the pinnulas, and one interior. These 

 leaflets are ciliated on their internal margin. 



b. Organs appertaining to the several segments. 



1. Anterior Cephalic Segment. — This segment presents, in its 

 front emargination, (A, fig. 1,) two minute rounded papillse, cov- 

 ered on their inner surface with very short hairs, which appear to 

 correspond to the inner antennas of other Crustacea. Below and 

 just behind their insertion we observe a small semicircular process 

 convex outward, which projects a short distance beyond the sur- 

 rounding surface. 



Toward the lateral extremity of this segment, on its lower sur- 

 face, there is a remarkable organ, which the animal employs in 

 attaching itself, (I, fig. 1 and fig. 19,) but which has heretofore 

 been considered its eyes. It consists of a thin nearly circular 



