GENERAL CONSIDERATIONS 



185 



sense organs and no muscles to be controlled, the useless 



O 



nervous system becomes very much simplified (Fig. 46). 



Apparently because of the protection thus afforded, the 

 Sacculina penetrates now within the tissues of the crab, 

 becoming an internal parasite instead of an external parasite 

 as at first (Plate 101, D). While thus parasitic it gets its 

 food from the blood of the crab, which of course contains 

 much digested food ready to be assimilated. As digested 

 food is supplied for its use, the Sacculina has no need of 



A B c 



FIG. 46. Development of Sacculina carcnn. 



A. Larva which has just become attached to the base of a hair (b) on the surface of a crab. 

 It is throwing off its legs and part of its body. B, C, D. Further stages in the degeneration of 

 the Sacculina larva while attached to the outer surface of the crab. 



digestive organs of its own, and consequently these dis- 

 appear. Here, within the tissues of its host, relieved of all 

 need of gathering or digesting its own food, and freed from 



O O O O 



the necessity of moving about from place to place by its own 

 energy, it has an abundant amount of energy to devote to its 

 growth and to the formation and maturing of its reproductive 

 elements. 



The Sacculina soon becomes little more than a bag of 



