| Mr Potts, Thompsonia, a little known Crustacean Parasite. 459 
themselves, are said to be distinct even in the internal stage. 
As in Thompsonia so here, the external sacs on each host are 
always in the same stage of development, so that the only alterna- 
tive to a theory of origin by internal budding from a single Cypris 
larva, advanced by Geoffrey Smith, is that of simultaneous fixation 
of a crowd of gregariously inclined larvae. But the phenomena 
are so similar in the two forms that I venture to think that the 
same broad explanation must cover both. The proof of a budding 
process in Thompsonia, which I offer here, is in my view important, 
if indirect, support for Geoffrey Smith’s theory for Peltogaster 
socialis. It would be absurd to suppose that this latter species is 
on the same line of descent as Thompsonia, but it shows that the - 
latter genus may well have had an ancestor with many external 
‘sacs of normal Rhizocephalan type. 
A full account of this form, and also of Hapalocarcinus, 
described in the next paper, will appear later in the publications 
of the Department of Marine Biology of the Carnegie Institute of 
Washington, and I am indebted to Dr A. G. Mayer for permission 
to publish these preliminary notes. 
| 
| 
| 
