z 
454 Mr Potts, Thompsonia, a little known Crustacean Parasite. — 
iS 
crab Thalamita prymna, strongly parasitised by a form resembling 
Thompsoma. An examination of the material from these two 
sources and a careful comparison with the published accounts 
of Coutiére and Hafele leads me to conclude that Coutiere’s genus ; 
should not be retained. | 
The cardinal point which comes from this research is that | 
previous investigators were entirely mistaken in their coneeption 
of the organisation and life history of the parasite. The reason | 
for this is to be found in the fact that they were unable to i 
examine living or well fixed material which is necessary for 
successfully tracing the course of the root system. ) 
Externally the parasite consists of a number of small eaternal 
sacs, sometimes as many as 200 on a single host, which a 
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Fie. 1. Abdominal appendage of Snntenbe infected by Thompsonia x 120. 
To show the branching root system connecting the very young external sacs. The 
specimen was fixed with corrosive sublimate and stained with borax carmine. | 
The roots therefore have taken up very little stain compared with the tissues of the 
host and are shown unshaded. v.m. visceral mass, m. mantle, p. peduncle, cl. 7 
blind branch of root system which after further ‘development will become an | 
external sac. A 
from the limbs, both thoracic and abdominal, occurring so thickly. 
as to seem to embarrass motion. Coutitre and Hafele regard | 
each of these external sacs as a separate individual with a root: 
system of its own, and derived from a Cypris larva. I claim, on 
the other hand, that the external sacs are all budded off from 
one root system continuous throughout the host, and so that each | 
host is parasitised by a single individual and not by a hundred or | 
more gregariously inclined Rhizocephalans. 4 
