AN EARTHWORM 247 



end the capsule begins to draw together at the ends, and 

 when this egg capsule (Fig. 130, A) has been pushed off it 

 is a short, spindle-shaped, tough-coated sac, about as large 

 as a grain of wheat. Inside the capsule each egg unites with 

 a sperm in the act of fertilization. 



Egg capsules are found buried in the soil and under stones 

 and boards and other protected places. They are often mis- 

 taken for seeds of some plant, which accounts for their iden- 

 tity being unknown to most people. 



Relation to Environment. We could find no animal better 

 than an earthworm to show how an organism may be adapted 

 to its environment. In the first place, the slender, rounded 

 body with its pointed anterior end is the best possible form 

 for movement through the soil, which is usually displaced 

 by muscular effort. The appendages, which in the earth- 

 worm's relatives are never hard enough to be used for dig- 

 ging, are here only minute bristles. The animal is thus not 

 impeded by useless organs. The smooth skin is thin, and 

 since it is kept moist it serves as a breathing surface. 

 Branched gills such as are developed on many relatives of 

 the earthworm that live in the sea, would in an earthworm 

 be not only superfluous but even much in the way. 



It is a wonderful fact that the earthworm has no eyes and 

 still is able to distinguish light from darkness ; for, except in 

 the time of a rainstorm or in case of disease, it never leaves 

 its burrow in the daytime. Earthworms not only know the 

 direction from which light comes but they crawl away from 

 light of high intensity and crawl toward light of low inten- 

 sity. In nature earthworms come out at night in the warm 

 season, probably in response to the stimulus of a low in- 

 tensity of light, but also for the purpose of obtaining food. 

 They often leave the tail firmly anchored in the mouth of 

 the burrow by means of the projecting bristles. When dis- 

 turbed they spring back into the opening with incredible 



