CHAPTER XIV 

 MORE ABOUT WHEEL ANIMALCULES 



MICROSCOPIC as the wheel animalcules are 

 they yet have been watched and examined by 

 their admirers to as great a point of intimacy as 

 that reached by the devotees of insects or of birds. A 

 remarkable fact about them is that in about 130 different 

 species (out of the 500 known) it has been found that the 

 males are diminutive creatures, about one-tenth the size 

 of the females, and devoid of digestive canal ; in fact, little 

 more than minute swimming sacs full of spermatozoa. 

 In one group, that of the crawling Rotifers, to which the 

 common wheel animalcule, figured in the last chapter, 

 belongs, no male at all has ever been discovered. They 

 are all females. They are precisely those wheel animal- 

 cules which are known to microscopists for their power of 

 surviving (like the little water-bears or tardigrades and 

 some other minute animalcules) the desiccation, or 

 " drying-up " of the water in which they were living, 

 swimming, and crawling (see Chapters XV. and XVI.). 

 And it is quite probable that this power of resistance 

 to the adverse conditions of changing seasons has, in 

 the crawling Rotifers, taken the place of the produc- 

 tion of eggs fertilized by a male. For, as in the case 

 of the crustacean water-fleas (and of the terrestrial 

 plant-lice, or aphides and gall-flies), it is found that the 

 female Rotifers or wheel animalcules, which hatch from 



fertilized eggs, are themselves " parthenogenetic," and lay 

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