M. Hesse on a new Parasitic Crustacean. 77 



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, which 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 ofi", 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 AnomicB, 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 simis 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 organ zed creatures : 

 it is probably also for the same reason that old and inferior individuals 

 are also attacked. 



