116 CRUSTACEA. 



ated leaflets. The palpi of the mandibles are always wanting. They 

 all have two eyes, situated in some at the extremity of two movable 

 pediclesj their antennse, but two in number in several, are generally 

 small and not fitted for natation. 



We will divide the Lophyropa into three principal and very natu- 

 ral groups, the two first of which approach the Crustacea of our 

 three first orders in their mandibles, each of which is furnished with 

 a palpus, and in some other characters. 



1. Those — Carcinoida, Lat. — whose more or less ovoid shell is 

 not doubled like that of a bivalve, and leaves the inferior portion of 

 the body exposed. They never have antennae resembling ramified 

 arms. They have ten feet, more or less cylindrical or setaceous. 

 The ova, in those females whose gestation has been observed, are 

 contained in two external sacs situated at the base of their tail. 

 Some of them have eyes. 



2. Those — OsTRAcoDA, Lat. 5 Ostrapoda, Straus — whose shell is 

 formed of two pieces or valves resembling those of a muscle, united 

 by a hinge, and closing while the body is quiescent. They have but 

 six feet(l}, neither of which terminates in a digitated fin, nor is ac- 

 companied by a branchial lamina. Their antennae are simple, fili- 

 form or setaceous. They never have more than one eye. Their 

 mandibles and superior jaws are furnished with a branchial leaf. 

 The ova are placed under the back. 



3. The last — Cladocera, Lat.; Daphnides, Straus — have but 

 one eye, and the shell doubled but without a hinge (Jurine), termi- 

 nating posteriorly in a point, und leaving the head, which is cover- 

 ed by a kind of shield like a rostrum, exposed. They have two, 

 usually very large, antennae, resembling arms, divided into two or 

 three branches directly above the peduncle, which are furnished 

 with threads, always projecting and serving as oars. Their ten 

 feet(2) are terminated by a digitated or pectinated fin accompanied, 

 the two first excepted, by a branchial lamina(3). 



Their ova are also placed under the back; their body always ter- 

 minates posteriorly in the manner of a tail, with two or three threads 

 at the end. The anterior extremity of the body is sometimes pro- 



_ _ . 



(i; According to Straus, the first pair of feet; but although these parts by serv- 

 ing- as oars perform their functions, I nevertheless consider them as analogous to 

 the lateral antennae of the superior Crustacea and to the two superior ones of a 

 Cyclops, which here also concur with the feet in producing locomotion. 



(2) MuUer gives eight to the Cytherese; reasoning from analogy, we may pre- 

 sume that he was mistaken. 



(3) This character applies especially to Daphnia, the most numerous subgenus 

 of this division, and by analogy, to Polyphemus and Lynceus. 



