METHOD OF OBTAINING INFUSORIA, ETC. 389 



be carried to the bottom of shallow ponds, and, whilst laid 

 horizontally, the surface of the mud should be scraped ; the 

 phial, when quite full, may be corked to prevent shaking ; at 

 home the mud may be put into a large jar, and filled up with 

 water, in a day or two the animalcules will have come to the 

 surface of the subsided mud, from which they may be taken 

 away quite clean by means of a fishing-tube. When water 

 has quite left a pond, a box or phial full of the surface mud 

 should be taken home and treated in the same way as the 

 more liquid kind above described. In order to preserve the 

 infusoria and other large animalcules at home, the conditions 

 under which they have been found should, in all cases, be 

 closely imitated. Such plants as will live in water without 

 much mould and not speedily decay should be selected, these 

 will all tend to keep the water healthy for the animalcules, and 

 also serve as food for them ; the plan adopted for preserving 

 the plants should be the same as that already alluded to in 

 page 380, the larva of the Ephemera and small snails being 

 employed to free them from confervse, which wiU be found to 

 interfere with the growth of most aquatic plants." 



Method of Obtaining and of Keeping Hydras. — One of the 

 most extraordinary aquatic animals that the microscopist is 

 likely to procure in his searchings in pools and ditches, is that 

 known as the Hydra, or fresh-water Polype ; it appears to 

 have been first noticed by Leeuwenhoek, in 1703, but it was 

 reserved for the inquiring genius of M. Trembley, then 

 residing at the Hague, in 1739, to discover its wonderful 

 powers of reproduction. In England there are as many as 

 four or five varieties, one of them is of a delicate green 

 colour, whilst the others are more or less yeUow or brown ; 

 each, when in an expanded state, consists of a long semi- 

 transparent tubular body, from one end of which protrude 

 several long delicate arms or tentacula, varying in number 

 in the diiFerent species, six or twelve generally being the two 

 extremes. Within these arms is a mouth capable of being 

 dilated, so as to receive animals nearly as large in size 

 as the Polype itself. In the contracted state, the animal 

 appears like a small round ball, the arms being drawn in so 



