THE EVIDENCE FROM DEGENERATION. 



351 



to pay for its free board and lodging ; and in this fashion Nature may 

 be said to revenge the host for the pains and troubles wherewith, like 

 the just of old, he may be tormented. 



Numerous life-histories testify clearly enough to the correctness 



FIG. 250. SACCULINA. FIG. 251. YOUNG SACCCLINA. 



of the foregoing observations. Take, as an example, the history ot 



Sacculina (Fig. 250), which exists as a bag-like growth attached 



to the bodies of hermit crabs, 



and sends root-like processes into 



the liver of its host. No sign of 



life exists in a sacculina beyond 



mere pulsation of the sac-like body, 



into and from which water flows by 



an aperture. Lay open this sac, and 



we shall find the animal to be a bag 



of eggs and nothing more. But 



trace the development of a single 



egg, and one may derive therefrom 



lessons concerning living beings at 



large, and open out issues which 



spread and extend far afield from 



sacculina and its kin. Each egg of 



the sac-like organism develops into 



a little active creature, possessing 



three pairs of legs, generally a single 



eye, but exhibiting no mouth or 



digestive system parasitism having 



affected the larva as well as the 



adult. Sooner or later, this larva 



known as the nauplius (Fig. 251) 



will develop a kind of bivalve shell ; 



the two hinder pairs of limbs are cast off and replaced by six pairs 



of short swimming feet ; whilst the front pair of limbs develops to 



form two elongated organs whereby the young sacculina will shortly 



attach itself to a crab "host." When the latter event happens the six 



FIG. 252. BARNACLES. 



