URINARY ORGANS OF THE AMPHIPODA. 
189 
suggest that these concretions have something to do with the 
process of “ casting the skin/' and that the first mentioned 
were animals in which this had just taken place, whilst the 
second were those in which preparation for it was being made. 
The concretions are, of course, extremely minute, and have 
only been obtained from a few specimens, so that it is not 
easy to determine exactly their nature. Distilled water does 
not dissolve them, nor is there any uric acid present, but I have 
been able to clearly detect phosphoric acid, and hence they 
seem to differ from those found by Nebeski in Orchestia 
cavimana, where he states that they consist of carbonate of 
lime. 
It has not been possible to observe what becomes of the 
concretions, but in one of the specimens mounted a few of 
them, whether accidentally or not I cannot say, have passed 
out into the alimentary canal by means of the mouth of the 
tube, showing, at all events, that it is perfectly possible for 
them to do so. 
At first sight it might appear as if these tubes were homo- 
logous with the Malpighian tubes of Tracheata, and until their 
development has been worked out and compared with that of 
the latter, it is impossible to settle the question definitely. In 
all specimens they arise from the point of junction of the mid 
and hind guts, but on close inspection and by means of sections, 
it can be demonstrated clearly, in at all events certain cases, 
that they belong really to the mid gut 
Thus fig. 4 represents a longitudinal vertical section through 
the hinder part of the alimentary canal of Gammarus pulex. 
One of the tubes is seen arising on the dorsal surface, but its 
lining epithelium is clearly continuous directly with that of 
the mid gut, whilst there is seen to be a distinct break 
(fig. 4, x ) where the latter ceases and the hind gut begins. 
These organs, which have such a strangely limited distri- 
bution amongst Crustacea are certainly, as is proved by their 
products, excretionary, and are very probably also urinary in 
function, but the knowledge which we at present possess of 
their point of origin from the alimentary canal prevents us 
