126 THE BOOK OF NATURE STUDY 



from our survey here. The same objection applies to the in- 

 teresting Rotifers, and to the whole of the Protozoa. The Coel- 

 entera, or hollow-bodied animals, are only feebly represented in 

 fresh water, but in spite of its small size a word or two may be 

 said about Hydra. Take a handful of water-weed from a pond 

 in summer-time, selecting especially ponds covered with duck- 

 weed ; or better, allow water and weed to flow in';D a wide- 

 mouthed bottle. Empty the contents of the bottle into a glass 

 vessel and put the vessel in a good light. You will quite pro- 

 bably see on the weed some tiny thread-like green creatures, 

 whose slender delicate bodies float out in the water, showing 

 the bunch of tentacles at the free extremity, and ever and again, 

 at the faintest alarm, cower down close to the weed to which 

 the other end is attached. This is hydra, an animal of very 

 interesting simplicity of structure, and one which has been the 

 subject of a great number of experiments. Without a lens one 

 can make very little of the structure, but it is quite visible to 

 the naked eye, and one can also see the green colour. Not all 

 may be green, however, for there is also a brown hydra in our 

 ponds. The point of greatest interest is that this little creature 

 is all that our ponds have to represent the sea-firs and jelly- 

 fish, the anemones and sea-pens which are so abundant in the 

 sea. 



As aids in identifying the animals of fresh water the following books of reference 

 maybe mentioned : Vol. viii. of the Cambridge Natural History, ''Amphibia and 

 Reptiles," by Gadow (London, 1901), gives an account of all our British 

 Amphibia. Fishes may be identified from Day's Fishes of Great Britain and 

 Ireland (London, 1880-4), which includes both marine and freshwater forms. 

 Molluscs, both marine and freshwater, are treated of in Forbes and Hanley's 

 History of British Mollusca (London, 1853), as well as in Jeffrey's British Conchology 

 (London, 1863-9). F r insects many books are available, both technical and 

 popular ; we may mention Wood's Insects at Home (London, 1883) as a simple 

 and interesting book, not costly, as are most of the more elaborate works mentioned 

 above. In the same author's Homes without Hands (London, 1889) there are 

 chapters on the habits, etc., of aquatic animals. The two volumes on " Insects" 

 in the Cambridge Natural History (vols. v. and vi., 1901) are full of interesting 

 facts, while both for insects and for other aquatic invertebrates the volume on 

 " Invertebrates" in the Royal Natural History (London, 1896) may be consulted. 

 This Natural History, which is not costly, despite its six volumes, is a good 

 book for a school reference library, and shows much more consistency of 



