21 G 
CLASS CRUSTACEA. 
This order in Linnaeus embraces the genus 
Oniscus, 
Which we shall divide into six sections. 
The first, Epicarides, Lat., is composed of parasite iso- 
pods, without eyes or antennae, the body of which is very flat, 
very small, and oblong in the males, much larger in the 
females, in the form of a narrowed oval, and a little curved 
posteriorly, hollow underneath, wdth a thoracic border, divided 
on each side into five membranaceous lobes. The feet are 
situated on this border, very small, curled up, and of no use 
either for walking or swimming. The under part of the tail is 
furnished with five pair of small ciliated imbricated leaflets, 
answering to as many segments, and disposed on two longitu- 
dinal ranges; but the posterior extremity is deprived of 
appendages. The mouth presents distinctly but two mem- 
branaceous leaflets, applied on another of the same consistence, 
in the form of a large quadrilateral figure. The lower conca- 
vity, forming a sort of flat basket, is filled by the eggs. 
These Crustacea form but a single subgenus, that of 
Bopyrus, Lair. ( Bopyrus crangorum, Lat.) 
The second section, CymothoADA, Lat. comprehends the 
isopods with four very apparent antennae, setaceous, and al- 
most always terminated by a pluri-articulated stem ; having 
eyes, a mouth composed as usual, vesicular gills, disposed 
longitudinally in pairs ; the tarsi formed oi from four to six 
segments, with a fin on each side near the end, and the ante- 
rior feet most usually terminated hy a strong claw or hook. 
These Crustacea are all parasites. 
Sometimes the eyes are carried upon tubercles, at the sum- 
mit of the head. The tail is composed of only four seg- 
ments. 
