CRUSTACEA. 



147 



a sliield, or the animal may be enclosed within a bivalve shell. To 

 this order belong the curious " Brine-shrimps " (Artemia), which are 

 found in the brine-pans of salt-works, or in salt lakes. 



Okders CliVDocera, Copepoda, and Ostracoda. 



These orders deserve mention more from the extreme abundance 

 of their commoner forms than for any other reason. They include 

 a number of minute Crustaceans, most of which are commoidy 

 called " Watei'-fleas," and which abound both in fresh and in salt 

 water, in almost all regions of the glolje. They are, however, 

 mostly so small that, though visible to the e}^e, they can only be 

 satisfactorily examined under the microscope. As an example of the 

 C'ladocera may be taken the " Branched-h(5rned Water-flea " (Daphn la 

 pulex, fig. 97, 6), thousands of which may be captured in any pond in 

 summer. In this pretty little species the whole body is enclosed in 

 a bivalve shell, which is so transparent Jhat the whole organisation 



-Frcsli-water Eutomostmca. a Cjii'V: 

 c l.'il'.'loiis quadrico 



of the animal is clearly visible through it. The h'jad is distinct, and 

 carries a single eye. The greater antemue are branched. The males 

 are smaller than the females, and nuich fewer in numlier ; and it 

 appears to be a well-estalilished fact that the female, when once 

 fei-tilised by the male, can not only lay eggs for the rest of her life, 

 but can transnut the power of producing fertile ova to her young 

 for several genei-ations. Of the i'ofjepnda one of the commonest is 

 the Ci/dops (fig. 97, c), in which the cei>halotli(>rax is covered by a 

 shield, and there is a well-developed abdomen. The female carries 

 on either side a kind of pouch or ovisac, in which the eggs remain 

 till they hatched. The little Ostracoda (tig. 97, a), are all minute 



