CLASS OF CBUSTACEA. 447 



3. The isopoda, in whom the abdomen is, on the contrary, 

 well developed, and carries beneath a series of false branchial 

 limbs. The anilocra (fig. 432), the sphaeroma, and the class 

 cloporte (oniscus) belong to this order. 



Fig. 434,. Cyclops. 



576. The branchiopoda, as we have already said, are 

 small Crustacea, whose limbs no longer serve for walking, but 

 assume the form of foliaceous plates, constituting at one and 

 the same time organs of natation and respiration. Such are the 

 limnadiae, which have been already mentioned (Fig. 433), the 

 apis, the branchipes, the daphnise. It is to this group that 

 the trilobites seem to have belonged : marine animals, whose 

 fossil remains are found in the most ancient strata of the 

 globe, but of which there exists not at present any living re- 

 presentative in the seas. 



577. The entomostraca are also formed only for 

 swimming, and in youth they all possess a certain number of 

 rigid double-oared limbs; but in the adult state they are 

 mostly sedentary, and then the body becomes deformed in a 

 very singular manner ; in general they have but a single eye, 

 placed in the middle of the forehead, and their respiration 

 seems to take place over the whole surface of the body. 



578. Some, called copepoda, are always very active, and 

 possess large antennas and a masticatory apparatus ; these are 

 the cyclops, or monocles (Pig. 434).* 



579. Others live as parasites, on fishes, Crustacea, &c., 

 and have the mouth elongated in the form of a proboscis or 

 beak, armed with style-formed appendages adapted to pierce 

 the integuments of the animals whose juices they suck. They 



* The Entomostraca play an important part in the great economy of 

 nature. They form the especial food of many valuable fishes of the family 

 saltnonidse, clupeadse, and corregoni ; and it is evident, from the remains in 

 the limestone strata, that they abounded in the seas and fresh waters of the 

 ancient world. fi. K. 



