248 CRUSTACEA. 



— particularly in the females — situated at the anterior and inferior 

 extremity of the head, which have but a single joint with one or two 

 setae at the extremity. In the 



Polyphemus, Miill., 



As in Daphniae and Lynceus, the antennae are in the form of oars 

 divided into two branches ; but each of them is composed of five 

 joints. The head, moreover, which is very distinct and rounded, is 

 provided with a sort of neck, and is almost entirely occupied by a 

 large eye. The feet are completely exposed. 



But a single species has hitherto been discovered, the Mono- 

 culuspediculus, L. ; Deg., Insect,, VII, xxviii, 6 — 13 ; Polyphe- 

 mus oculus, Miill., Entom.,xx, 1 — 5 : Cephaloculus stagnorum. 

 Lam. ; Jurine, Monoc, xv, 1 — 3 ; Desmar., Consid., LIV, 1, 2. 

 The feet, according to Jurine, have no resemblance whatever to 

 the Monoculi of this division. They consist of a thigh, leg, and a 

 tarsus composed of two joints, from the extremity of which, that of 

 the last pair excepted, issue several small threads. Two small antennae, 

 consisting of a single joint and terminated by two threads, project 

 from the anterior extremity of the head. The shell is so extremely 

 diaphanous, that all the viscera can be distinguished, The matrix, 

 when filled with eggs, occupies the greater part of its interior. 

 Their greatest number never exceeds ten. In following the gradual 

 developement of the fcetus, we are struck with the early appearance of 

 the eye, in comparison with that of other parts of the body. It is 

 greenish at first, and passes insensibly to a deep black. The abdo- 

 men, after being flexed from behind forwards, bends suddenly back- 

 wards to form a long, slender, pointed tail, from which issue two 

 long articulated thieads. The animal always swims on its back, and 

 most frequently in a horizontal direction, by the quick and repeated 

 motion of its arms and feet, and executes all sorts of evolutions with 

 ease and agility. When young, and after its first changes, it is sub- 

 ject to a disease called the ephippium*; but this ephippium or saddle 

 ahvays has a determinate figure, and never contains the two oval 

 ampullae observed in the Daphniae. These animals do not live long 

 in a state of captivity, nor can their young ones be raised, at least 

 such was the case with Jurine, who could not preserve them after 

 their first changes. Among all the specimens which were the subjects 

 of his observations, he could not find a single male, though, it is true, 

 he could procure but very few of them, this species being rare in the 

 environs of Geneva. It is said, however, to be very common in the 

 marshes and ponds of the north, where it aggregates in considerable 

 numbers. In the 



Daphnia, Miill., 



The oars arc always exposed to their base or to the origin of their 

 peduncle ; they are as long, or almost as long as the body, and are 

 divided into two branches, the posterior of which consists of four 

 joints, the first very short, and the other, or the anterior, of three. 



* Sec the following article, Duphnia, r- 250. 



