240 CRUSTACEA. 



2. Those OSTRACODA, Lat. ; 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 *, neither of which terminates in a digitated fin, nor is accom- 

 panied by a branchial lamina. Their antennse are simple, filiform 

 or setaceous. They never have more than one eye. Their mandi- 

 bles 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, and leaving the head, which is covered 

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

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

 brandies directly above the peduncle, which are furnished with 

 threads, always projecting and serving as oars. Their ten feet f are 

 terminated by a digitated or pectinated fin accompanied, the two first 

 excepted, by a branchial lamina J. 



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- 

 longed into a kind of rostrum, and at others forms a kind of head, 

 almost entirely occupied by a large eye. 



The first division of the Lophyropa Branchiopoda that of the 

 Carcinoida may be divided into two according to the number of the 

 eyes. 



Some of them have two. 



Here the shell completely invests the thorax ; the eyes are large 

 and very distinct, and the intermediate antennae are terminated by 

 two threads. 



ZOEA, Bosc. 



Very large globular eyes completely exposed, and horn-like pro- 

 jections on the thorax. 



Zoeapelagica, Bosc., Hist, Nat. Crust. II, xv, 3, 4. The 

 body semi-diaphanous ; four antennae inserted under the eyes, 

 the external ones bent into an elbow and bifid ; a kind of long 

 rostrum on the forepart of the thorax and between the eyes, 

 and a long pointed prominence on the posterior part of the 

 back. The feet are very short and hardly visible, the two last 

 excepted, which are elongated or terminate in a fin. The tail 

 is as long as the thorax, curved, and formed of five joints, the last 

 being large, crescent- shaped and spinous. It was discovered by 

 Bosc in the Atlantic Ocean. 



* 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. 



f Muller gives eight to the Cytherese ; reasoning from analogy, we may presume 

 that he was mistaken. 



J This character applies especially to Daphnia, the most numerous subgenus of 

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



