346 - APPLIED BIOLOGY 



( 



by the fact that the worms are sometimes seen wriggling in 

 troughs where horses drink, and where horsehairs may also 

 be seen. But that there is no connection between the worms 

 and the hairs can be demonstrated by any one who will 

 place in bottles, stoppered with cotton, one or a thousand 

 horsehairs, and await developments. 



Scientific men long ago studied the structure and embry- 

 ology of horsehair worms and solved the mystery of their 

 appearance in pools, watering troughs, etc. The structure 

 of the worm is essentially the same as that of the other 

 round worms. Its scientific name is Gordius, in allusion to 

 its habit of twisting into a tangle like the famous Gordian 

 knot which Alexander the Great cut with his sword. Its 

 eggs develop into minute larvae, which become parasites in 

 insects, fishes, frogs, and other animals. Later the parasites 

 escape from these animals and develop into horsehair worms. 

 Those seen in horse-troughs have completed their life-history 

 in insects. 



299. Spontaneous Generation. The belief that a living 

 worm might develop from a dead horsehair is opposed to 

 the commonly accepted idea that living things develop only 

 from similar living things. However, this idea that all life 

 comes from life (biogenesis) is rather recent. In former times, 

 even scientific men believed in spontaneous generation of life 

 in dead, or not-living matter. The sudden appearance of 

 numerous earthworms, frogs, mice, insects, etc., was ex- 

 plained by assuming that they had suddenly originated spon- 

 taneously. When accurate studies of life-histories began to 

 be made, it soon became evident that all the larger animals 

 and plants originate only from organisms like themselves. 

 Until 1638, it was supposed that maggots developed spon- 

 taneously from putrid meat ; but in that year an investiga- 

 tor showed that maggots never appear on meat which is 

 screened so as to keep flies from laying eggs on it. Studies 

 of habits have shown that tfce sudden appearance of earth- 



