556 



THE MICROSCOPE. 



puts on somewhat the appearance of the Lobster crab 

 (Galathea), and after another step attains its true crab- 

 form, being the highest development of which it is capable. 

 These changes are not produced gradually, but by a suc- 

 cession of " moults," the animal becoming at times sluggish, 

 casting its hard covering, and reappearing in a new guise. 

 The after growth of a crustacean is carried on by the system 

 of moulting ; the hard calcareous case of the animal pre- 

 venting its growth in any other mode. And as in the 

 higher orders of Crustacea, so also amongsb the Entomos- 

 traca, transformations of this kind constantly take place. 

 Cyclops quadricornis, when first born, is totally unlike its 

 parents, being of an ovoid shape, having only two shoit 

 antenna? and two pairs of feet ; in three moults the 

 animal reaches its perfect form, with its two pairs of an- 

 tennse, five pairs of feet, and body divided into several 

 distinct rings or segments. 



The animals comprising the order Ostracoda are generally 

 of very minute size ; the body, 

 which strongly resembles that 

 of the Copepoda, is always 

 enclosed in a little bivalve 

 shell, the feet and antenna) 

 being protruded between the 

 lower edges of the valves. 

 These little shells so closely 

 resemble those of minute 

 bivalve Mollusca, that those 

 of some of the larger species 

 have actually been described 

 by conchologists as the cover- 

 ing of animals belonging to 

 that class. The antennae are 

 often curiously branched ; and 

 the hinder extremity is usu- 

 ally prolonged into a sort of 

 tail, which is seen in constant 

 action when the animal is in 

 motion. In Cypridina, the 

 body is entirely enclosed by 

 a shell, of which the genus Cypris (fig. 255) is an example ; 



Fig. 255. 



1, Cypris. 2, Polyphemus, Cyclops. 

 3, Branchiopus stagnalis. 



