EARTHWORMS, LEECHES, AND SEAWORMS 87 



met with segmented bodies. Moreover, these animals 

 have a double nerve running throughout the whole length 

 of the bodj' and a distinct brain. Correlated with the 

 brain, as we should expect, are eyes, at least, in some of 

 the leeches and seaworms. 



Adaptations to mode of living. — The home of the earth- 

 worm is in the soil. It burrows all through the soil in 

 search of its food. The food is derived largely from 

 quantities of soil, which are passed straight through the 

 ahmentary canal. To facilitate the passage of such a 

 quantity of useless material the ahmentary canal is 

 straight. At the same time there must be a large amount 

 of absorbing surface in the canal to take up the food 

 contained in the soil, while it is passing through. To 

 provide such a surface and yet not have the canal coiled, 

 the body must be long, and so it is. More than this, the 

 shape of the body is most economical for a burrowing 

 animal without strong jaws or claws. That is, if the body 

 of the worm were short and thick, a larger hole would have 

 to be made and, consequently, more digging done. Again 

 there are many setse on the ventral side of the body which aid 

 greatly in crawling up the sides of the burrow. 



The leech is furnished with suckers, which certainly 

 adapt it to its mode of life. Moreover, the crop is some- 

 times enlarged by lateral pouches, that it may hold suffi- 

 cient food to last the animal for a long time. Finally, the 

 body can be greatly distended to provide room for the 

 storage of food. Undoubtedly leeches often go several 

 weeks without finding an animal whose blood may be 

 drawn. If it were not for the enlarged crop and extensible 

 body, in which the blood can be stored to tide over the in- 

 tervals when no food is obtainable, the animal might die. 



