On the Rhizocephalan Genus Thompsonia, etc. 25 



well prove that the slitting up of a single central tumour into the rudiments of 

 several individuals takes place at a still earlier stage, before any differentiation 

 of the tumour has occurred, possibly soon after the entrance of the embryonic 

 cells of the Cypris larva. The final test of this hypothesis must, however, be 

 left to the future, for someone who has the opportunity of studying either 

 this species or some such form as Thylacoplethus, where material is abundant."* 



I have given this quotation from Smith in full because I feel certain 

 of the general correctness of his suggestion that where " numerous 

 individuals on a single host"f occur in the Rhizocephala they are really 

 produced from a single Cypris larva by a process of budding. It is 

 amply proved in Thompsonia, I claim, by the evidence here put for- 

 ward, even though I have not been able to describe the endoparasitic 

 stages; and in Peltogaster socialis, with which the passage I quote is 

 specially concerned, I feel confident that a similar phenomenon takes 

 place. 



It is perfectly certain, however, that the details of the budding pro- 

 cess differ in the two genera. Smith has shown in P. socialis, there are, 

 in the endoparasitic stage, separate central tumours (the anlagen of 

 the visceral mass) and separate root systems belonging to them. 

 He suggests that division of the original embryo may even take place 

 before there has been any differentiation. Thus early division and 

 consequent development of separate embryos, each with a separate 

 central tumour and root system, are probably characteristic of the 

 budding process in P. socialis. In Thompsonia the parasitic organism 

 is at all stages continuous within the body of the host and consists of 

 a single root system which is established first, and a large number of 

 reproductive sacs which are budded off and become external at a later 

 phase. Their number and late and recurrent appearance are character- 

 istic of Thompsonia and correlated with a degenerate structure. 



It will be of interest in connection with this discussion to give a small 

 series of drawings of a social Rhizocephalan very similar to Peltogaster 

 socialis and, I think, identical with the form from Japan described by 

 Kriiger as a new genus and species (Peltogasterella socialis). In 1911, 

 while enjoying the delightful hospitality of Professor Trevor Kincaid 

 at the Puget Sound Marine Biological Laboratory at Friday Harbour, 

 I collected a number of individuals of the hermit crab, Pagurus alas- 

 kensis heavily infected by this form. Plate 2 represents the para- 

 site at three widely different periods of growth, but in each case all the 

 individuals are of uniform size, arid, it can hardly be doubted, of the 

 same age too. The only alternative to a theory of budding is that of 

 simultaneous fixation of the Cypris larvae. This theory indeed sup- 



*Smith, 1. c., pp. 57-58. 



fThis does not apply to all the numerous instances where a crab bears two or three SacculincR 

 or a hermit crab more than one Peltogaster. Far more commonly these must be cases of suc- 

 cessive infections. 



