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 larve, 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. 
