228 
CRUSTACEA. 
to breathe, and to preserve their branchiae in a proper state for the 
exercise of that function. 
Tins order according to the system of Linnaeus embraces the genus 
Ontscus, Lin,, 
Which we will divide into six sections. 
The first — E pica rides, Latr. — is composed of parasitical Isopoda, 
with neither eyes nor antennoe, 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 ])arts distinctlv vlsihlc in the mouth are two mem- 
branous leaflets laid upon anotlier 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 alwavs 
found the individual presumed to be the male. Its extreme smallness 
seems to forbid all ]) 0 ssibility of copulation; according to Desmarest 
it is provided with tvro eyes; its body is straight and almost linear. 
These CrustaQea form but a single subgenus, that of 
Bopyrus, Lat., 
The most common species is the Bopyrus crangorum, Lat., 
Gener. Crust, et Insect., I, 114; 3[onocuim crangonwi, Fah. ; 
Fouger. de Bondar, Mem. de I’Acad. Roy. des Sc., 1772, pi. 1 ; 
D esmar. Consid. XLIX, 8 — 13. It lives on the Palcemon 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 s])ecics, the B. des jjaienions, has Itcen 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 antenme, almost universally terminated 
by a pluri-articulatcd 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 arc all parasitical. 
'I’he eyes are sometimes placed on tubercles on the top of the 
bead ; the tail consists of but four segments. 
♦ See the work of Desmarest, who has completely described this subgenus, 
t See our general observations on the Malacostraca with sessile eyes. 
