CRUSTACEA. 147 



a shield, 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. 



Orders Cladocera, 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 commonly 

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

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

 mostly so small that, though visible to the eye, they can only be 

 satisfactorily examined under the microscope. As an example of the 

 Cladocera may be taken the " Branched-horned Water-flea" {Daphnia 

 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 that the whole organisation 



Fig. AT. — Fresh-water Entomostraca. a Cypris tris-striata; b Dajphnia pulex ; 

 c Cyclops guadricornis. 



of the animal is clearly visible through it. The head is distinct, and 

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

 are smaller than the females, and much fewer in number ; and it 

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

 fertilised by the male, can not. only lay eggs for the rest of her life, 

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

 for several generations. Of the Copepoda one of the commonest is 

 the Cyclops (fig. 97, c), in which the cephalothorax 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 (fig. 97, a), are all minute 



