EARTHWORMS, LEECHES, AND SEAWOKMS 



77 



inclosed by the Ix)(ly walls tliat is terine<l the l.)0(Ay cavity, 

 or ccelome. In the earthwoi-iu, as we have ali-eaily noted, 

 the body cavity is divided into conipartinents by thin cross 

 partitions (Fig. 39). 



Structure of the body walls. — The body walls of an earth- 

 worm are niatle up of five distinct layers of tissue. The outer- 



Doi bed blood 

 le^sel 



uticle 



Epidermis 



-Circular 

 >^^m muscles 



-^ ilongitiidirial 



' ■ '• ' muscles 



^■^TJCoelomic 

 i>:i V epithelium. 



Ventral blood Nerve cord Setae 

 vessel 



Fig. 40. — Diagram of a. cross section of an earthworm. 



most layer is a thin, transparent meuilirane called the cuticle. 

 Just beneath this is the skin, composed of long cells jjlaced 

 vertically to the surface of the body like the |)alisade cells 

 of a plant leaf. Next to the skin is a layer of muscles that 

 encircle the body, hence called circuJnr muscles. Within 

 this layer is one composed of muscles that run lengthwise 

 of the body, therefore known as the longituclinal muscles. 

 Lastly is a very thin layer of flat cells callerl the ccelomic 

 epithelium. This layer of cells hues the ccelomic cavity. 



