282 APPENDIX. 



summit of the cone, technically the 'chimney,' just described. This 

 motion in the transparent fluid would be quite unperceived but for an 

 occasional larger particle, which may be seen to come wandering 

 along beneath the covering film until, reaching the ' chimney,' it is 

 suddenly puffed out as from a volcano. If the young sponge is very 

 carefully fed with finely powdered carmine, or other suitable (in- 

 soluble) coloring matter, the course of circulation may more readily 

 be followed." EDWARD POTTS, in The Microscope, vol. x. p. 163. 



VI. On Hydra Culture. The hydra lives in shallow ponds and per- 

 manent pools in still water. In warm weather it is often found near 

 the surface, attached to the stems of reeds, or pendent from the lo\ver 

 surface of floating leaves. In winter it will more often be found 

 attached to leaves, that have fallen on the bottom. Its narrow, cylin- 

 drical body is about half an inch long, its tentacles are of equal length, 

 and its color is pale brown, or, in another species, clear green. The 

 leaves, etc., should be drawn out of the water, and their surfaces 

 examined with a lens. The hydras, if present, will be much con- 

 tracted after being disturbed ; but the tentacles do not entirely dis- 

 appear when contracted, but remain as a circle of fleshy knobs about 

 the free end of the hydra, and these will serve for recognition. If 

 the leaves be placed in a jar of water, the hydras will extend them- 

 selves again in a few minutes. 



Perhaps the surest way to find them is to place the leaves, etc., in a 

 shallow white dish or plate in water for a few minutes. If present, 

 they will extend themselves when quiet, and may easily be seen upon 

 a white background. Though not abundant anywhere, there is hardly 

 a locality that will not furnish a good supply after a proper search. 

 They are very apt to be found year after year in the same places. 



Hydras for class use should be collected a week or more before they 

 are to be used. They may be kept in a jar of water, or aquarium, in 

 which some small aquatic plants are placed. They will need to be 

 supplied with food. They eat readily small crustaceans not larger 

 than cyclops, daphnia, etc. ; also protozoans and microscopic plants. 

 If the jar in which they are to be kept be taken to the field and filled 

 with the water in which they are found, and a few of the plants to 

 which they adhere, sufficient food to keep them for a long time will prob- 

 ably be taken up with the water and the plants. None of the larger 

 animals (e.g., snails) which feed on hydras should be left in the jar. 



Many of the hydras will attach themselves to the sides of the jar, 

 on the side toward the light. When wanted for study, they may be 

 dislodged by pushing a knife blade between their bases and the glass, 

 and taken up with a dropping tube. 



