326 
CLASS CRUSTACEA. 
ancl leaves the lower part of the body uncovered. They never 
have antennae in the form of ramified arms. Their feet are ten 
in number, and more or less cylindrical or setaceous. The 
females, whose gestation has been observed, carry their eggs 
in two species of external sacs, situated at the base of the 
tail. Some have two eyes. 
2nd. Those (Ostracoda, Latr .) ostrapoda , Straus, whose 
testa is formed of two pieces, or valves, resembling those of 
the shell of a muscle, united by a hinge, and enclosing the 
body in a state of inaction. They have but six feet, none of 
which terminate in the manner of a digitated fin, accompanied 
with a branchial plate. Their antennae are simple, filiform, 
or setaceous. They never have more than one eye. Their 
mandibles and upper jaws are provided with a branchial 
plate. The eggs are placed under the back. 
3rd. The last, (Cladocera, Lai Daphnides , Straus,) have 
also but a single eye, and the testa folded doubly, but with- 
out a hinge (J urine), terminated posteriorly in a point, and 
leaving the head, which is covered with a sort of buckler in 
the manner of a beak, exposed. They have two antennae, 
usually very large, in the form of arms, divided into two or 
three branches, at the end of the pedicle, furnished with 
filaments, always projecting, and serving the purpose of oars. 
Their feet, ten in number, are terminated by a digitated, or 
pectinated fin, and accompanied, with the exception of the 
first two, by a branchial plate. The character is particularly 
applicable to daphnia, the most numerous subgenus of this 
division, and by analogy, to the polyphemi, and lyncoei. 
Their eggs are also situated under the back. Their body 
is always terminated posteriorly, in the manner of a tail, with 
two setae or filaments at the end. The anterior extremity of the 
body is sometimes elongated into a beak, and sometimes forms 
an approximation to a head, almost entirely occupied by a 
large eye. 
