30 Papers from the Department of Marine Biology. 



It is, I think, very difficult to establish specific distinctions until 

 more material of Thompsonia has been collected and compared. At 

 present one can not say how far the character of the host affects such 

 superficial characters as size, shape, and colour. I think that in the 

 three forms which I have examined we very possibly have three distinct 

 species, but I refrain from burdening systematic zoology with fresh 

 names when we know so little about the validity of those already 

 given. 



One point in Coutiere's description of Thylacoplethus may have an 

 important bearing on the systematic question. In the three Alpheids 

 which he examined the parasites were, he says, situated on "les 

 quatres premiers pleosternites, qui se montrent souleves en un large 

 bourrelet transversal." And he contrasts this with the form, certainly 

 very near and probably identical,* which Spence Bate mentions and 

 figures in the Challenger monograph on the Decapoda, occurring to the 

 number of 30 individuals at the base of the abdominal appendages of 

 Alpheus malleodigitatus from Fiji. The parasites which I describe from 

 Synalpheus are also confined to the appendages and are therefore, if I 

 read Coutiere's account aright, much more like the Challenger form than 

 are those of the French investigator. It is possible that a section of the 

 genus is characterised by the occurrence of the external sacs on the 

 sternum rather than on the appendages. 



THE INFLUENCE OF THE PARASITE ON THE HOST. 



So far as my material goes, the effect of the parasite upon the host is 

 negligible. The gonad does not diminish in size and in some cases at 

 least does not cease to function. One female Alpheid which I collected 

 had both fertilised eggs and parasites upon her abdominal appendages. 

 This was probably an exceptional case. 



The secondary sexual characters do not undergo any change. The 

 specimen of Thalamita prymna was a heavily parasitised male, but the 

 abdomen showed no broadening, assumption of female appendages, 

 or diminution of copulatory appendages. In the Alpheid Synalpheus 

 brucei the abdominal appendages do not differ in the two sexes and the 

 female is to be distinguished from the male only by the greater width 

 of the abdomen and better development of the pleura. This character 

 is not affected by the parasite. 



*Spence Bate did not think that hia para3ite was related to Sacculina because the embryo 

 exhibited no Crustacean affinities. Development must have been at a very early stage ; the 

 general appearance and position are so unmistakably those of Thompsonia. 



