Notes on Animals in small Aquaria. 4535 



extraordinary circumstances, as the following observations will tend 

 to show : — In a small aquarium that had had gold-fish kept in it for a 

 length of time, but which had been removed into one of larger dimen- 

 sions and more fully exposed to the light, an enormous number of the 

 Hydra fusca were observed to have made their appearance very soon 

 after this removal. Wishing therefore to ascertain if the appearance 

 and rapid increase of these polypes had been prevented by the gold- 

 fish, the following experiments were made : — Fifteen individuals of 

 the Hydra fusca were placed in the aquarium containing the gold-fish, 

 but they very soon disappeared, having, I presume, been devoured by 

 the fish ; a second fifteen were then introduced, but with the same 

 result. At the same time as this experiment was made, fifteen Hydras 

 were placed in a tank containing four minnows {Leuciscus Phoxinus) 

 and a pair of small eels, but as the minnows did not appear to touch 

 them, the same number of polypes being counted over several times 

 during a period of three weeks, they were soon forgotten altogether. 

 After a space of about seven months had elapsed from this time, the 

 minnows were observed to assume a most extraordinary aspect, the 

 head appeared very much swollen, and the eyes of all of them looked 

 as though starting out of their heads, being forced upwards and in an 

 outward direction and much enlarged ; by degrees the gills of some 

 of them became streaked with bloody markings, and this gradually 

 extended to the base of the pectoral fins. The whole appearance 

 was most distressing to contemplate, particularly as it was im- 

 possible, from ignorance of the cause, to adopt any remedial 

 measures. Judging from their appearance my impression was 

 that they had been poisoned, and, assuming that it must have arisen 

 from something putrescent which they might have raked out of the 

 materials at the bottom of the aquarium, the whole of the water w r as 

 drawn off clear by a syphon, the gravel and sand thoroughly washed, 

 and everything replaced in the tank with the fish ; no improvement, 

 however, appeared to follow, the fish got worse and ultimately died. 

 This was in June, 1853. Before this extraordinary change came on, 

 the fish had been observed to cluster together in one particular 

 secluded spot, and rarely came out as they had been accustomed to 

 do, and when they did venture forth they rubbed or jerked themselves 

 with much force against the gravel and rock-work, as though some- 

 thing was irritating the skin : nothing, however, was visible. I had 

 had these fish for about eighteen months in the same aquarium. As 

 the water was perfectly bright and clear, and free from all odour or 

 unpleasant taste, I procured six fresh minnows and placed them in 



