THE WORMS 69 



the greater number of species the development is round- 

 about, and one or more hosts are inhabited before the young 

 assume the adult condition. Such is the case with the 

 dreaded Trichina (Fig. 37, B), which infests the bodies of 

 several animals, particularly the rat. When these forms 

 are introduced into the alimentary canal of the rat, for 

 example, they soon lay a vast quantity of eggs, sometimes 

 many millions, which develop into young that bore their 

 way into the muscles of the body, where they may remain 

 coiled up for years. If the body of the rat be eaten by some 

 carnivorous animal, these excessively small young are lib- 

 erated during the process of digestion and rapidly assume 

 the adult condition in the alimentary canal, likewise giving 

 rise to young which pursue again the same course of de- 

 velopment. 



Another example of a complicated life history is in 

 the Gordius or " horsehair snake " (a true worm and not a 

 snake) frequently seen in the spring in pools where it lays 

 its eggs. These eggs develop into young which bore their 

 way into different insect larvae, which are in turn eaten by 

 some spider or beetle, and the worm thus transferred to a 

 new host. In this they grow to a considerable size, and 

 then make their exit from the body of the host and finally 

 become adult. 



64. Spontaneous generation. The ancients believed that 

 many animals were spontaneously generated. The early 

 naturalists thought that flies arose by spontaneous genera- 

 tion from the decaying matter of dead animals ; from a 

 dead horse come myriads of maggots which change into 

 flesh flies. Frogs and many insects were thought to be 

 generated spontaneously from mud. Eels were thought to 

 arise from the slime rubbed from the skin of fishes. Aris- 

 totle, the Greek philosopher, who was the greatest of the 

 ancient naturalists, expresses these beliefs in his books. It 

 was not until the middle of the seventeenth century 

 Aristotle lived three hundred and fifty years before the 



