EARTHWORMS, LEECHES, AND SEA WORMS 



77 



inclosed by the body walls that is termed the body cavity, 

 or ccelome. In the earthworm, as we have already noted, 

 the body cavity is divided into compartments by thin cross 

 partitions (Fig. 39). 



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

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



Dorsal blood 

 -'' vessel 



Cuticle 



Epidermis 



« Circular 

 *! muscles 



iLongitudinal 

 muscles 



^^f-Co^lomic 

 c)ntheUunt 



Ventral blood Nerve cord Setae 

 vessel 



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



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

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

 vertically to the surface of the body hke the palisade cells 

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

 encircle the body, hence called circular muscles. Within 

 this layer is one composed of muscles that run lengthwise 

 of the body, therefore known as the longitudinal muscles. 

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

 epithelium. This layer of cells lines the ccelomic cavity. 



