754 THE MICROSCOPE. 



visitation of a peculiar fever was clearly traceable to 

 the presence of Paramcecium (fig. 6) ; and numerous 

 deaths were attributable to them. Frogs, newts, and 

 other aquatic animals have been killed by these noxious 

 creatures. The water of the Firth of Forth is 

 frequently seen, in summer time, deeply coloured by 

 moving masses of minute crustaceans. To some rivers 

 and seas they impart a deep red colour. The rate of 

 increase of daphnia and cyclops is truly surprising. 

 Such is their amazing fecundity, as estimated by the 

 late Dr. Baird, that a single pair of Cyclops <juadri- 

 cornis will, in the course of six months, produce a 

 progeny numbering four billions five hundred millions 

 (4,500,000,000). A large number of species of ento- 

 mostraca are parasitic on marine and freshwater fish. 

 It is contended, however, that they attack the sickly, 

 and so make way for the " survival of the fittest." 

 Probably this is so, unless it will be conceded that 

 the sickness of the fish is a consequence of the presence 

 of the parasite. Fish thus afflicted are said, by fisher- 

 men, to be "lousy." A vegetable parasite, the Sapro- 

 legnia, is the cause of the Salmon disease, producing 

 a sort of leprosy over the body of the fish. 



Thames water favours the presence of hydroid 

 polyps ; consequently, various species of hydra may be 

 found adhering to pieces of weed and decaying vege- 

 table substances. Fig. 5 is a full-grown Hydra fusca. 

 Starch granules and epithelium (fig. 11) are held in sus- 

 pension, and nearly always form a small portion of the 

 sediment of sewage-polluted river- water. Such bodies 

 can in no way be estimated by chemical analysis, as they 

 only form a minute part of any residual ash. Starch 

 enters largely into the food of animals, and it may be 

 assumed that this albuminoid product must have been 

 conveyed into river- water in the excreta of animals. 



The microscopical analysis of water is a large and 

 very important one, and those of my readers who wish 

 for further information on the subject will do well to 

 consult Dr. J. D. Macdonald's " Guide to the Micro- 

 scopical Examination of Drinking Water," published 

 by Churchill. 



