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 Coutiére’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 pléosternites, qui se montrent soulevés 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 Coutiére’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 
brucet 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 his parasite 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. 
