

CRUSTACEA. 113 



vision for growth, as the rigidity of the encasing 

 armour forbids the possibility of increase in its 

 capacity. The growth, therefore, is periodic : at 

 certain intervals the hard crust is thrown off in 

 several pieces, a new crust having been prepared 

 beneath, which is at first soft, flexible, and expan- 

 sile ; the body, now freed, instantly enlarges in all 

 directions, and in a few minutes has attained the 

 full extent of growth needed ; the crust at once 

 hardens, and in a brief space becomes as inflexible 

 as was its predecessor, admitting no further en- 

 largement either of its own surface, or of the con- 

 tained organs. The animal usually undergoes this 

 process in the most retired situation it can find, 

 instinctively conscious of its unprotected condition 

 when soft, and apparently feeling sick and feeble. 



Among the most singular creatures of this great 

 Class are those comprised in the Orders Epizoa 

 and Entomostraca. The former exhibit shapes 

 the most bizarre, so that the young student can 

 scarcely believe that they are animals at all. 

 They are parasitic in their habits, usually living 

 attached to the gills, or to the interior of the mouth 

 of fishes ; and to fit them for this mode of life, they 

 are furnished with strange organs of many forms. 

 It is necessary that they should be able so firmly 

 to adhere to their supporters as to resist the force 

 of all ordinary influences to dislodge them ; and 

 many are, in fact, so anchored in the flesh, that in 

 order to preserve the epizoon, we are compelled to 

 cut out the surrounding parts of the fish. Some- 

 times this is effected by an array of hooked fangs 

 and pincer-like claws, combined with sucking- 

 disks, as in Caligus, while a slender tube pierces 

 the flesh and pumps up the vital juices ; some- 

 I 



