62 PRIMITIVE ANIMALS 



Barnacle. In the Rhizocephala, on the other hand, 

 the Cypris larva attaches itself by its antenna to a 

 hair on the surface of a crab and passes into the crab 

 a small group of cells which multiply rapidly in the 

 blood-cavities of the host and finally produce the 

 root-system and tumour-like body of the adult parasite 

 which is thrust to the exterior (Fig. 12 A and B). 

 The Cypris larva itself, which has thus infected the 

 crab with a small portion of its body, does not play 

 any further part but drops off dead. From our 

 knowledge of the life-history of the Rhizocephala, 

 therefore, we are in a position to affirm that these 

 peculiar animals are really Barnacles which instead of 

 fixing upon inanimate objects have taken to attaching 

 themselves to living crabs and finally to infecting 

 them and living as internal parasites within them. 

 Since we have made the acquaintance of the 

 Nauplius larva, we may consider further the signifi- 

 cance of this larval form in the Crustacea generally. 

 We meet with it in nearly all the orders of the small, 

 lower group of Crustacea, the Entomostraca ; the 

 majority of these animals hatch out from the egg as a 

 Nauplius, which by the gradual addition of segments 

 behind becomes transformed into the adult animal. 

 In the majority of the higher Crustacea, the Mala- 

 costraca, the development of the egg, before hatching, 

 proceeds to a later stage, the Nauplius stage with 

 three appendages being passed through within the 



