148 INTRODUCTION TO ZOOLOGY. 



Additional Facts About the Eartliwonn. 



"We find in the earthworm no lungs, gills, or other 

 breathing organs. It is very evident that the worm 

 must breathe; that is, there must be an excliange 

 of CO2 for oxygen. This exchange of gases takes 

 place directly through the skin. The skin is a thin 

 membrane which is kept moist by the conditions in 

 which the animal lives. It is in effect a thin, moist 

 membrane, with blood loaded with CO2 on one side, 

 and with oxygen on the other. These are the only 

 conditions necessary in a breathing organ, and are the 

 conditions attained in the construction of lungs and 

 gills. 



This fact enables us to account for some of the 

 habits of earthworms. Earthworms seldom come out 

 of their burrows in the daytime, except when it is 

 raining. At such a time, they often appear in great 

 numbers and in unexpected places. Some people be- 

 lieve that they rain down. They neither rain down 

 nor are drowned out. The rain and moist atmosphere 

 enables them to move freely on the surface of the 

 ground without danger of suffocation. Often, at the 

 close of the shower, worms are stranded on pavements 

 or other places into which they cannot burrow, and so 

 perish. 



Should earthworms come out of their burrows in 

 daytime when the air is dry, the skin would quickly 

 become dry and they would be in danger of perishing. 

 At night, the danger from this source is much less, 

 and this sufficiently accounts for their activity at night 

 and apparent quiescence in daytime. Although they 

 have no eyes, Mr. DarAvin found reason to believe 

 that they could tell the difference between daylight 

 and dark, by means of the first two segments. 



We take the earthworm to represent the branch of 

 Vermes. It is probably as good a representative as we 



