M. Hesse on a new Parasitic Crustacean. V7 
conditions—that is to say, exclusively upon young fishes*, always 
of the same species and at the same place, and generally on the 
right side, never hitherto on both sides. It occurred to us to 
open one of these tumours, in the expectation of finding some 
Entozoa in it, in a cyst or scirrous tube; but what was our 
surprise when we found that we had to do with a being which, 
although belonging to a higher class, must none the less take a 
place among the lowest ranks of the Crustacea with which it 
might be classed. 
This tumour is of about the size of a lentil, sometimes smaller, 
rarely larger. Two, or even three, may often be seen upon the 
same fish; but in this case one is always stronger than the 
others, and this is usually the first one, that is to say, the nearest 
to the spot above indicated. On carefully examining the tumour, 
it is seen to be formed by a certain quantity of scales, whjch are 
deranged from their usual symmetrical arrangement, convergent 
towards a centre, and superposed upon each other in such a 
manner as to form a projecting and culminating point, at the 
central summit of which we see a small round hole, evidently 
opened to establish an issue. 
By carefully removing the lateral scales which are at the base 
of the cone, and have undergone no alteration, we arrive at those 
which, on the contrary, have undergone a modification. These, 
which are much thicker than the others, are, so to speak, 
double-bottomed. If they are torn off, we see, on looking at 
them from below, that they present a comparatively spacious 
cavity existing between the upper wall, which is concave, and 
the lower one, which is flat—an arrangement greatly resembling 
that of the shells of the Anomia, if it were not that in these 
Mollusca the two valves can open and close by means of the 
hinge placed at the upper part of the shell, whilst in the case 
now before us these two surfaces are soldered together, and are 
consequently immoveable. 
The scales which have undergone the modifications just men- 
tioned present below (that is to say, on their flat surface) two 
vertical perforations, of which that placed near the lower margin 
is always the largest ; we also generally detect some other small 
holes pierced in the upper surface. These scales are applied to 
the side of the fish by the flat side. When we tear them off, we 
see at the spot which exactly corresponds with the large perfo- 
ration above mentioned, a hollow or sinus pierced in the flesh, 
* We have already had occasion to indicate that it is especially to young 
fishes that parasites attach themselves, probably as being less able to free 
themselves. This law appears to be general for all organized creatures : 
it is probably also for the same reason that old and inferior individuals 
are also attacked. 
