138 HISTORY OF CRUSTACEA. CHAP. XII. 



has certainly nothing improbable about it. If then the 

 cement-ducts of such a Cirripede instead of merely 

 spreading on the surface, pierced or pushed before 

 them the soft ventral skin and penetrated into the inte- 

 rior of the host, this must have been beneficial to the 

 animal, because it would be thereby more securely 

 attached and protected from being thrown off during 

 the moulting of its host. Variations in this direction 

 were preserved as advantageous. 



But as soon as the cement-ducts penetrated into the 

 body-cavity of the host and were bathed by its fluids, 

 an endosmotic interchange must necessarily have been 

 set up between the materials dissolved in these fluids 

 and in the contents of the cement-ducts, and this inter- 

 change could not be without influence upon the nourish- 

 ment of the parasite. The new source of nourishment 

 opened up in this manner was, as constantly flowing, 

 more certain than that offered by the nourishment 

 accidentally whirled into the mouth of the sedentary 

 animal. The individuals favoured in the development 

 of the cement-ducts now converted into nutriferous 

 roots, had more than others the prospect of abundant 

 food, of vigorous growth, and of producing a numerous 

 progeny. With the further development, assisted by 

 natural selection, of the roots embracing the intestine 

 of the host and spreading amongst its hepatic tubes, the 

 introduction of nourishment through the mouth and all 

 the parts implicated in it, such as the whirling cirri, the 

 buccal organs, and the intestine, gradually lost their 

 importance, became aborted by disuse, and finally dis- 



