486 
CRUSTACEA. 
Genus 8. — TROCHETIA.— Lamarck. 
Generic Character .—Body oblong, anteriorly cylindrical, pos¬ 
teriorly broader, and subdepressed ; with a contractile disk; a 
large circular ring, a third from the anterior end of the body; 
mouth with two lips, the upper one large and obtuse ; without 
teeth, jaws, or eyes; vent above, near the posterior disk. 
Genus 9. — HIRUDO.— Linnceus. 
Generic Character .—Body oblong, blunt, somewhat depressed, 
posteriorly widened, and composed of many contractile seg¬ 
ments ; a prehensile disk at the posterior extremity; mouth 
naked, dilatable, with three internal teeth or horny jaws; no 
eyes ; vent above, near the posterior disk. 
CLASS VI.—CRUSTACEA. 
Animals without vertebral column, having generally a crusta- 
ceous covering, more or less solid ; with the members of the body 
articulated, and provided with distinct organs of circulation ; re¬ 
spiring by means of branchiae—This class consists of nine 
orders. 
ORDER I—DECAPODA. 
With the branchiae of a pyramidal shape in the form of leaf¬ 
lets, situated near the last four feet-jaws, and of the feet, con¬ 
cealed under the sides of the shell; head not distinct from the 
trunk of the body. This order contains seventy-six genera. 
Section I.— Brachyura. 
The branchiae consisting of numerous small leaflets, sur¬ 
mounting one another, and placed along a common axis, seven 
on each side ; post-abdomen folded or bent beneath, and gene¬ 
rally received into a cavity beneath the pre-abdomen, and not 
terminated by a fin. 
