THE MICROSCOPE IN ZOOLOGY. 173 



The family of Entomostraca contains a number of gen- 

 era, nearly all of which are but just visible to the naked 

 eye. They are distinguished by the inclosure of the body 

 in a horny or shelly case, often resembling a bivalve shell, 

 though sometimes of a single piece. The tribe of Lophy- 

 ropoda (bristly -footed), or " water-fleas," is divided into 

 two orders, the first of which, Ostracoda, is characterized 

 by a bivalve shell, a small number of legs, and the absence 

 of an external ovary. A familiar member of this order, 

 the little Cypris, is common in pools and streams, and may 

 be recognized by its two pairs of antennse, the first of 

 which is jointed and tufted, while the second is directed 

 downwards like legs. It has two pairs of legs, the poste- 

 rior of which do not appear outside the shell. 



The order Copepoda has a jointed shell, like a buckler, 

 almost inclosing the head and thorax. To this belongs 

 the genus Cyclops (named from its single eye), the female 

 of which carries on either side of the abdomen an egg 

 capsule, or external ovarium, in which the ova undergo 

 their earlier stages of development (Plate XVI, Fig. 130). 



The Daphnia pulex, or arborescent water-flea, belongs to 

 the order Cladocera and tribe Branchiopoda. The other 

 order of this tribe, the Phyllopoda, has the body divided 

 into segments, furnished with leaf-like members or " fin 

 feet." 



When first hatched, the larval Entomostraca differ 

 greatly from the adult. The larval forms of higher 

 Crustacea resemble adult Entomostraca. 



The suctorial Crustacea, order Siphonostoma, are gener- 

 ally parasitic, mostly affixed to the gills of fishes by means 

 of hooks, arms, or suckers, arising from or consisting of 

 modified foot-jaws. The transformations in this order, as 

 in the Lerncea, seem to be a process of degradation. The 

 young comes from the egg as active as the young of 

 Cyclops, which they resemble, and pass through a series 

 of metamorphoses in which they cast off their locomotive 



