326 CLASS CRUSTACEA. 



and 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, Lat., 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 lyncaei. 



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. ,. 



