CRUSTACKA. 
248 
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- 
culus pediculus, L. ; Deg., Insect,, VII, xxviii, 6 — 13 ; Pobjpke- 
mus oculus. Mull., Entom.,xx, I — 5 : Cephaloculus stagoiorum, 
Lam. ; Jurine, Monoc., xv, I — 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 foetus, 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 thi-eads. 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 
always 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 Avith Jurine, Avho could not preserve them after 
their first changes. Among all the specimens Avhich Avere the subjects 
of his observations, he could not find a single male, though, it is true, 
he covdd procure but very fcAv 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, Avhere it aggregates in considerable 
numbers. In the 
Daphnia, MiilL, 
The oars are 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 tAVo branches, the posterior of Avhich consists of four 
joints, the first very short, and the other, or the anterior, of three. 
* See the following article, Daphnia, p. 250. 
