162 Proceedings of the Royal Physical Society. 
When examined with a high power these bodies are seen to have the 
shape of rice-grains, being short cylinders about three times as long as they 
are thick, with smooth contour and rounded ends. They are of uniform 
size. Though I have not actually measured them, I should say their leneth 
is ahput equal to the diameter of a human red blood corpuscle. | 
My first idea, judging from their occurrence in the blood and their 
smooth contour, was that they might be particles of fatty nature. They fail, 
however, to stain with osmic acid, with Scharlach R., or with Sudan III. They 
are readily fixed with corrosive sublimate or with formalin, and stain with 
basic aniline dyes. On subsequent differentiation with alcohol I have 
thought that I could see a nucleus in them. In addition to this their fixed 
shape and uniform size suggest that they are organised structures. The only 
feasible view with regard to them was that they were organisms of some 
kind, which, having gained access to the animal in some way, had multiplied 
in the blood. Against this view, however, was the apparent healthiness of the 
animals under such an overwhelming infection. 
Quite recently I have found that Vejdovsky (16, 17) has described, in 
spirit specimens of Gammarus zschokkei (from Garschina Lake, Switzerland), 
the occurrence of a “gigantic” nucleate micro-organism, which he has named 
Bactercum gammari. The bacterium occurs in enormous numbers in cells 
of the body itself and also free in the blood, undergoing division by mitosis.: 
Mencl (18), having re-stained Vejdovsky’s specimens by the iron-alum 
method, has published a minute description of the microscopic appearances 
of the organism. Perusal of these accounts leaves little doubt in my mind 
that the particles which I have seen in Gammarus marinus are micro- 
organisms similar to, if not actually identical with, Bacterrwm gammare. 
One interesting fact in connection with the infected animals is the 
disappearance of all pigment from the plasma in advanced stages of the 
disease. In brown-coloured Gammari with brownish plasma I have found 
these organisms present, yet few in number. When they are very abundant 
the plasma is colourless. Evidently the presence of the bacterium causes — 
some change in metabolism, whereby already-formed blood pigment is 
destroyed or the manufacture of new pigment is inhibited. Heim (13) has 
directed attention to the occasional occurrence of white crayfish; it is 
possible that these animals suffer from an organismal infection similar to that — 
of the Gammari. 
SUMMARY. 
1. Members of the Gammaridea owe their body colour in large degree to 
the colour of the blood; isopods in much less degree. 
