216 BIOLOGY 



hastens the development, since its rate is dependent on tem- 

 perature. Our common frog lays its eggs in irregular masses, 

 which may be found in abundance in the spring months around 

 pools of fresh water. The toad has somewhat similar breeding 

 habits, but lays its eggs in long strings. Inside the jelly the 

 fertilization of the eggs is completed and the development 

 begins, and here the young remain until they are ready to hatch 

 as young larvae. 



PHYSIOLOGY OF THE EARTHWORM 



The organs of the earthworm are much simpler than those of 

 the frog. Some of the systems of organs found in the frog are 

 apparently absent in the earthworm. There are, for example, 

 no lungs nor other special organs devoted to respiration; there 

 is neither heart nor system of bones for support. But although 

 some of these systems of organs appear to be absent, their 

 functions are not lacking. In other words, the earthworm has 

 exactly the same functions of life as the frog, but carries them 

 out in a simpler way. Respiration is carried on through the 

 skin; the motions of the animals are confined to a writhing 

 motion made by the muscles of the body wall ; the circulation 

 of the blood is produced by the contraction of the blood vessels 

 instead of by a heart; excretions are carried on through the skin 

 and also by the nephridia. In short, the earthworm has the 

 same general functions as the frog, only they are carried out on 

 a simpler scale and by a simpler series of organs. Since its 

 organs are simpler, we speak of the earthworm as having a 

 lower organization than the frog. 



