178 ON KEEPING MEDUSAE ALIVE IN AN AQUARIUM. 



The siphon to empty the bucket must be of large bore, so as to 

 carry away the water faster than it comes in. As soon as the siphon 

 has stopped running the bucket begins to fill, and when the weight of 

 the bucket exceeds that of the plunger, the bucket slowly goes down, 

 and the plunger comes up to the surface. The length of the stroke is 

 regulated by two stops, which prevent the beam moving too far up or 

 down. The plunger remains at the surface until the water has reached 

 the top of the siphon, and directly the siphon begins to act the bucket 

 is quickly emptied, and the plunger goes down nearly to the bottom. I 

 found that one down-stroke in eighty seconds was sufficient, and 

 regulated the apparatus so that the down-stroke was a little faster than 

 the up-stroke. 



The top of the bell-jar has a wooden cover with a narrow slit cut in 

 the centre to act as a guide to the plunger-rod, and to prevent the plate 

 knocking against the bell-jar on the downward plunge. 



The movement of the plunger produces numerous eddies in the 

 water, which are rendered visible by the movements of the copepods 

 and the medusae. The medusae are carried from one side of the bell- 

 jar to the other, or from the bottom to the top. This movement the 

 medusae appear thoroughly to enjoy, and during the intervals in which 

 the plunger is at rest they may be seen either taking a swim or floating 

 with their tentacles expanded, or else playing an active copepod, caught 

 on the end of a tentacle, as skilfully as an expert angler plays a large 

 fish. 



The first plunger bell-jar was started in the Plymouth Laboratory on 

 4th of September. (Fig. 1.) The bell-jar contained about ten gallons of 

 water, which had been in it about three months, and the glass was well 

 coated with algae. In this bell-jar were placed at intervals different 

 species of medusae, and a good food-supply consisting of copepods, 

 crustacean larvae, &c. I kept this bell-jar under close observation until 

 the 9th of October, when my visit to Plymouth terminated. The 

 temperature of the water was often taken, especially on hot days, and 

 occasionally the specific gravity. The temperature varied from 14-75° C. 

 to 17'5''C., and it was kept down on hot days by placing round the 

 outside of the bell-jar a strip of flannel, upon which played a jet of 

 fresh water. This acted very well, for when the temperature of the 

 room was about 21° C, the water in the bell-jar remained about 1G° C. 



All the species of medusae placed in this bell-jar not only lived 

 longer, but were in a better condition than if kept in still water. Some 

 species lived longer than others, which tends to show that much has yet 

 to be learnt on keeping medusae. Perhaps for some species a slow 

 revolving current would be better ; it could easily be obtained by turn- 

 ing a screw-propeller in the water. 



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