4536 Notes on Animals in small Aquaria. 



the tank ; for about ten days they appeared to be pretty healthy : 

 they did not, however, swim about freely, but herded together in one 

 corner of the aquarium, and then the same extraordinary change 

 gradually came on which had been observed in the others, and after 

 lingering for seventeen days they all died. On carefully scrutinizing 

 the different parts of the tank with a magnifying glass, my attention 

 was at once arrested by observing the enormous number of the Hydra 

 fusca which were present, particularly on the parts of the aquarium 

 where the fish had been accustomed tc-feed; that is, along the water- 

 line towards the light, at the base of the plants of Vallisneria spiralis, 

 about half-an-inch above the gravel, and on the whole of the rock-work 

 around the space where the minnows delighted to hide : here they 

 might have been seen stretching out from the sides, hanging down 

 from the top — in fact, in every possible direction : here, then, was a 

 solution of all the evil. It now became a question how these pests 

 were to be eradicated, and after canvassing in my mind a variety of 

 suggestions, I determined to endeavour to remove them individually, 

 and by this means they were speedily got rid of; from fifty to a 

 hundred being taken out daily. The method by which this operation 

 was effected was as follows: — A long glass capillary tube open at 

 both ends was introduced into the water, having the finger kept 

 tightly over the upper orifice, while, with the edge of the lower 

 opening, the polype was detached from its hold; the moment this 

 was effected and the Hydra began slowly to fall through the water, the 

 finger was removed, and the water with the polype was thus rapidly 

 driven into the tube by the pressure of the external column of water; 

 on replacing the finger the contained water and polype were removed. 

 By persevering in this course they were caught with the greatest 

 rapidity and dropped into another vessel before they had time to 

 attach themselves to the interior of the tube, falling through the water 

 like a miniature parachute. When situated in places where this mode 

 of capture could not be employed, as on the leaves of the Vallisneria 

 or on the under sides of the rock-work, they were pulled off with a jerk 

 by means of a small pair of forceps. In this manner between four and 

 five hundred polypes were removed from a small aquarium holding 

 about six gallons of water. Since this some small carp and also 

 minnows have been placed in the same water, and have continued 

 now for upwards of sixteen months in perfect health. 



It is a curious problem as to the manner in which this destruction 

 of life was brought about: my own impression is that the Hydras 

 seized on the minnows whenever their extended tentacula were 



