PARASITIC CRUSTACEA. 



207 



from independent predatory genera to others occasion- 

 ally parasitic, and again from these to others which 

 on leaving the egg immediately attach themselves for 

 life. Here, as elsewhere, parasitism seems an adaptation 

 to new habitats, which is recorded in the biography of 

 the individual with a reminiscence of the previous form. 

 The circumstances of the parasitic worms are repeated 

 by the parasitic Crustacea, as, moreover, a probably 

 primordial form of the crab family is preserved in 

 the metamorphoses of several orders of this large and 

 diversified, though coherent class. The larva, which, 

 it may safely be assumed, 

 approximates closely to 

 the primordial form, was 

 at one time taken for an 

 independent genus and re- 

 ceived the name of Nau- 

 plias. Hence a Nauplius 

 phase is spoken of, which 

 obtains especially among 

 the lower Crustacea, the 

 Copepoda, parasitical Crus- 

 tacea and Cirripedes, and 



the remarkable Rhizopoda connected with them ; but 

 is not wanting in the highest order, the decapodous 

 stalk-eyed crab. We shall later have to miake acquaint- 

 ance with the so-called curtailed development v/hich 

 among the crabs has been adopted by the decapods, and 

 it was formerly supposed by all. Were this actually 

 the case, we should still, by analogy, infer their connec- 

 tion with the other orders repeating the Nauplius phase 

 in the course of their development; but it was a welcome 

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