Chapter 4. 



The Endoparasitic Development of the Rhizocephala. 



Contents. 



1. The History and Development of Sacculina interna. 



A. Sacculina interna migrans. 



B. Sacculina interna. Organogeny and Evagination. 



C. Orientation of the body during organogeny. 



2. The Endoparasitic development of Peltoyaster curvatus. 



3. The Endoparasitic development of Peltogaster socialis, and Polyembryony in Sacculina. 



4. The general nature of the endoparasitic development of the Rhizocephala. 



5. The morphology of Ditplorbis and the derivation of the endoparasitic development. 



I. The History and Development of Sacculina interna. 



Those who have read Delage's memoir (1) and have followed the subsequent contro- 

 versy on the internal parasitism of Sacculina will remember that Delage describes no stage 

 between the indefinitely located fixation of the Cypris larva, and the appearance of the nearly 

 adult body of the Sacculina at the jiosition of evagination. At this latter stage the root system 

 is already fully etablished; it is apparent therefore that there is a wide gap between these 

 two stages of Delage. It was the existence of this gap which gave Giard's scepticism some 

 sort of basis. I am now able substantially to fill in that gap, and the earlier stages of the 

 internal parasite, which I have found, completely confirm Delage's deduction that there is an 

 internal stage of the life-history when the parasite is engaged in establishing the system of 



