54 ANIMAL FORMS 



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. 32, 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 this same course of 

 development. 



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 larva?, 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. 



54. Spontaneous generation. It is only within compara- 

 tively recent years that such life histories have been under- 

 stood. Formerly the sudden appearance of these and other 

 forms in various situations were accounted for on the ground 

 that they arose spontaneously without the intervention of 

 any living creature. Even yet we hear of the transforma- 

 tion of horsehairs into hairworms, and of frogs, earthworms, 

 and several other animals from inorganic matter, but such 

 assertions are based on superficial observations, and at the 

 present time no exception is known to the law that living 

 creatures arise from preexisting living parents. " All life 

 from life " (omnium vivum ex vivo) is a universal law. 



