OYPIUS LARVA OF SACCULINA CARCINI. 625 



The Fixation of the Cypris Larva of Sacculina 



carcini (Thompson) upon its Host, 



Carcinus msenas. 



By 



CieolTrey Siiiitli, M.A., 



New College, Oxford. 



With G Text-figures. 



The fixation of the Cypris iarva of Sacculina has hitherto 

 been successfully observed only by Delage (1), and since a 

 considerable degree of doubt has been thrown on his account 

 it may be of interest to record that I have been able this 

 autumn, at Plymouth, to rear the larvEe and observe all 

 the stages of fixation upon their host. My inability to obtain 

 the Cypris larvas of any of the Rhizocephala at Naples had 

 left a gap in uiy observations, which I am now in a position 

 to fill, and thus I am able to present a complete account of the 

 life-history of Sacculina from my own observations (2). 



It is extremely easy to obtain large batches of the Nauplii 

 of Sacculina or Peltogaster by selecting a crab whose para- 

 site is nearly ready to emit its Nauplii from the brood pouch, 

 and keeping it for a few days in a vessel of sea water. The 

 readiness of the parasite to emit the Nauplii can be judged 

 in Sacculiua by the purple colour of the mantle, and in Pelto- 

 gaster by the mantle becoming a paler pink than usual. 



The Nauplii of Peltogaster are not actively heliotropic, but 

 they are negatively geotropic, and in a short time all of them 

 reach the surface film of the water and perish, and I have 

 been unable to obviate this difficulty in rearing the larva3. 

 The Nauplii of Sacculina are actively heliotropic, but there is 

 no marked negative geotropism ; nevertheless, with the 



