24 Papers from the Department of Marine Biology. 
THE SOCIAL FORMS OF PELTOGASTER AND THOMPSONIA 
COMPARED. 
“ Peltogaster socialis . . . is remarkable for the fact that it is never found 
solitary, but always infests a single host in numbers varying between 4 and 30, 
the usual number being about 20 . . . The numerous parasites affixed to 
each host are always at very much the same stage of development, so that the 
infection by so many individuals must have taken place at the same time. 
There is a certain mystery to be solved here, because the parasite in general is 
so rare that its occurrence, when it does occur in such large numbers on a 
single host, must either mean a most peculiar gregarious habit in the Cypris 
larvee or else we must look for some quite different explanation. The explana- 
tion which occurred to me was that the numerous individuals on a single 
host are really the product of a single Cypris larva by a process of budding 
from the endoparasitic central tumour and its root system. Although this 
would mean an unique process in Crustacea, namely, the production of a true 
colony by budding, there is nothing inherently improbable in the hypothesis, 
if we take into account the peculiar nature of the development of the Rhizo- 
cephala, 7. e., the assumption in the middle of the developmental history of 
an embryonic condition. 
“There is also a further fact which made me expect to find such a process of 
budding. Delage, in his memoir, makes mention (p. 665) of finding in the 
central tumour of a Sacculina interna two cellular masses, representing the 
future visceral mass and mantle, instead of one, and he wonders whether it is 
possible for a single tumour ever to give rise to two Sacculine; but he dismisses 
the idea partly because his preparation was a poor one and partly because 
this hypothesis is contrary to the general facts of development. 
‘Now, in the course of my investigations on Sacculina interna, I have found 
incontestable evidence on two occasions that Delage’s first opinion is perfectly 
correct, and that occasionally two Sacculinze may begin to form a single central 
tumour; but whether two such Sacculine ever come to maturity I am unable to 
say. One of these specimens is shown in plate 6, fig. 10. It is here seen that 
two mantle and visceral masses are developing opposite one another in a 
single central tumour, which must of course have been produced from a single 
Cypris larva. 
“There is therefore a tendency towards a kind of polyembryony or budding 
of the ‘nucleus’ of the central tumour in Sacculina, and this led me to suspect 
that possibly in Peltogaster socialis, and in such a genus as Thylacoplethus 
(Coutiére), in which the parasite is present to the number of about a hundred, 
this process of budding has become normal and permanent. 
“‘ According to this view it should be possible to prove that the root systems 
of the apparently separate individuals of P. soczalis on the same host are all 
in connection; I was therefore at pains to investigate this subject. To my 
great disappointment I was able to prove that each individual has a separate 
root system at no point in continuity with that of another individual. Fur- 
thermore, in one instance I was so fortunate as to come across a crab infected 
with Peltogaster socialis, the individuals of which were still internal and at a 
very early stage in development, and even at this early stage there is no con- 
tinuity between the tumours and root systems of separate individuals. Plate 
6, fig. 9, is a section taken through two central tumours of P. socialis . . 
The hypothesis, therefore, that the individuals of P. socialis are produced by 
budding from a single tumour receives no confirmation so far from the inves- 
tigation of facts, but I do not yet altogether give up the hypothesis. It may 
