228 CRUSTACEA. 



to breathe, and to preserve their branchiae in a proper state for the 

 exercise of that function. 



This order according to the system of Linnaeus embraces the genus 



ONISCUS, Lin., 



Which we will divide into six sections. 



The first EPICARIDES, Latr. is composed of parasitical Isopoda, 

 with neither eyes nor antennae, the body of which, in the male, is 

 very flat, small and oblong ; much larger in the female, and having 

 an oval form narrowed and slightly curved posteriorly, hollow 

 beneath, with a thoracic border divided on each side into five mem- 

 branous lobes. The feet are placed on this border and cannot be 

 used either for locomotion or natation. The under surface of the 

 tail is provided with five pairs of small, ciliated, imbricated leaflets, 

 corresponding to as many segments, and arranged in two longitudi- 

 nal series ; there is no appendage, however, to the posterior extre- 

 mity. The only parts distinctly visible in the mouth are two mem- 

 branous leaflets laid upon another of the same nature, forming a large 

 quadrilateral figure. The inferior concavity forming a sort of 

 shallow basket, is filled with the ova. Near their outlet is always 

 found the individual presumed to be the male. Its extreme smallness 

 seems to forbid all possibility of copulation ; according to Desmarest 

 it is provided with two eyes ; its body is straight and almost linear. 



These Crustacea form but a single subgenus, that of 



BOPYRUS, Lat., 



The most common species is the Bopyrus crangorum, Lat., 

 Gener. Crust, et Insect., I, 114; Monoculus crangorum, Fab. ; 

 Fouger. de Bondar, Mem. de 1'Acad. Roy. des Sc., 1772, pi. 1 ; 

 Desmar. Consid. XLTX, 8 13. It lives on the Palaemon ser- 

 ratus, and the Pal. squilla, placed directly on the branchiae and 

 under the shell ; it occasions a tumour on one of its sides, re- 

 sembling a wen. The fishermen of the British channel con- 

 sider them as very young Soles or Plaice. 



A second species, the B. des palemons^ has been described by 

 Risso, under the female of which he observed eight or nine 

 hundred living young ones *. 



The second section CYMOTHOADA, Lat. comprises Isopoda with 

 four very apparent setaceous antennae, almost universally terminated 

 by a pluri-articulated stem ; having eyes, a mouth composed as 

 usual f; vesicular branchiae arranged longitudinally and in pairs; 

 the tail formed of from four to six segments, with a fin on each side 

 near the end ; and the anterior feet usually terminated by a small 

 stout nail or claw. They are all parasitical. 



The eyes are sometimes placed on tubercles on the top of the 

 head ; the tail consists of but four segments. 



* See the work of Desmarest, who has completely described this subgenus. 

 f See our general observations on the Malacostraca with sessile eyes. 



