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 

 larvse 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, i. 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 Sacculina}; 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 Sacculinse may begin to form a single central 

 tumour; but whether two such Sacculinse 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 

 (Coutiere), 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. socialis 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 



