io8 THE BOOK OF NATURE STUDY 



in width. An operculum is present as in Bithynia. The com- 

 monest species is P. vivipara (see Fig. 46). 



Leaving the freshwater molluscs, we come next to the 

 Arthropods, or jointed-footed animals, in which the body is 

 clothed with a thick coat or cuticle. This coat is so thick that 

 the limbs could not be moved were it not for the joints or soft 

 places, which permit of free movement. For our purposes we 

 may regard the Arthropods as consisting of three classes, the 

 crabs, shrimps and their allies forming the CRUSTACEA, the 

 INSECTS, some of whom are aquatic during at least part of their 

 life, and the ARACHNIDS, including the spiders, mites and scor- 

 pions of warm countries. Of these the insects will largely pre- 

 dominate in the freshwater aquarium, for whereas the Crustacea 

 furnish some of the most interesting inhabitants of the marine 

 aquarium, there are but few which can be utilised in the fresh- 

 water tanks. 



The most interesting of the freshwater Crustacea is un- 

 doubtedly the crayfish (Astacus\ very common in parts of 

 England. It is, however, a form naturally inhabiting swift- 

 flowing streams, and therefore difficult to keep alive in captivity 

 unless the tank has a more or less constant flow of water through 

 it. As a schoolroom tank is not likely to fulfil this requirement 

 we must leave the crayfish out of account. 



The next two common Crustacea are hardy enough, but 

 they are carrion feeders and not particularly attractive. It is 

 a bad sign if they flourish well in an aquarium, for this means 

 that the tank contains much decomposing vegetable and animal 

 matter, which is not a condition favourable to the continued 

 existence of other more interesting animals. These two animals 

 are Asellus aquations, sometimes called the water-hog louse, 

 and Gammarus pulex, often incorrectly called a freshwater shrimp. 

 It is much nearer to the sandhoppers of the seashore than to 

 the true shrimps. In Asellus the body is flattened from above 

 downwards, and the animal closely resembles the wood-lice found 

 beneath decaying wood, or in the dark corners of the greenhouse. 

 It reaches a length of about \ inch, and may be found 

 creeping over the weeds at the bottom of ponds. The sessile 

 eyes are a notable distinction from Crustacea like crayfishes, in 



