THE MICROSCOPE AS A RECREATION 



133 



Fig. 129. — ^Holopedium. 



there is a history in an abbreviated form of the stages through 

 which the species originally developed. The so-called water- 

 fleas, for instance, are little crustaceans like small shrimps. 

 They are hatched out 

 from eggs as small oval 

 bodies with short legs, and 

 very little else except one 

 eye. After a time the 

 young creature casts ofi 

 its skin and becomes rather 

 more elaborate in form. 

 This goes on stage by stage 

 till it develops into a 

 creature with the most 

 complete series of. legs, 

 antennae, tail, and other 

 appendages. It has as- 

 sumed the appearance of a small shrimp. In some species it 

 goes further, and after having for a short time lived a free and 

 energetic life it develops into nothing but a bag and suckers, 

 which attach themselves to fishes and suck their nutriment from 

 the fish's body. 



This points to a degeneration in the development which has 

 taken place in the history of a race who found it less fatiguing, 

 if less honourable, to live on other people rather than to fight 

 their own battle in life. 



These small Crustacea, generally known as water-fleas, are 

 one of the chief foods of fish, both salt 

 and fresh water. They exist in such 

 enormous numbers in some parts that 

 they even satisfy the appetite of the 

 whale. The sea is sometimes of a blood- 

 red colour due to the myriads of a 

 coloured form of these creatures. There 

 is no pond that has not many varieties, 

 and they can be best captured with a 

 collecting net. Certain forms are phos- 

 phorescent, but aU are more or less trans- 

 parent, and can be thoroughly investi- 

 gated under the microscope. Fig. 129 

 shows one form found in the lakes of 

 Cumberland, which is supposed to be a 

 delicacy beloved by the salmon trout and 

 the char. This curious species is em- 

 bedded in an envelope of jeUy much 

 larger than itself. It is quite transparent. The rolling of its 

 single eye, the beating of its heart, and the digestion of its food, 

 can all be watched under quite a low-power object glass. 



Fio.130. — Bytho- 

 trepes. 



