126 CRUSTACEA. 



the anterior and inferior extremity of the head which have but a 

 single joint with one or two setae at the extremity. In the 



PoLTrHEMUS, Mull. 



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; Polyphe- 

 mus oculus, Mull. , 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 development 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 abdomen, after being flexed from behind forwards, bends sud- 

 denly backwards to form a long, slender, pointed tail, from which 

 issue two long articulated threads. 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 subject to a disease called the ephippium(l); but this 

 ephippium or saddle always has a determinate figure, and never con- 

 tains the two oval ampullae observed in the Daphniae. These ani- 

 mals 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, 



(1) See the following article, Daphnia, p. 128. 



