140 ANNELIDA [CH. 



body can be pushed forward and- to which the hinder end of the 

 body can be drawn up. 



The colour and thickness of the body from the thirty-second to 

 the thirty-seventh segment dift'er in adult worms from those of the 

 segments which lie before and behind this band. This is due to 

 the presence in this region of certain ectodermal glands whose secre- 

 tion forms the cocoons in which the eggs are laid. This region of 

 the body is called the clitellum (xxxn xxxvn, Fig. 59). 



The surface of the body of an earthworm is glistening and 

 somewhat slippery. This is due to the cuticle, which is a thin 

 membrane secreted by the ectoderm cells of the skin ; if a dead 

 earthworm be soaked in water for a few hours the cuticle can be 

 easily stripped off the body. As we have already seen a similar 

 cuticle is present in Trematoda, Cestoda, Nematoda and Rotifera, 

 but in the two groups last mentioned it is much harder and forms 

 an external protective skeleton; even in the earthworm, where it is 

 soft, it acts as a protection to the underlying cells, and its smooth 

 surface enables the worm . to creep into narrow holes without 

 hindrance. The chaetae are simply large local thickenings of the 

 cuticle: they protrude from pockets called chaeta-sacs, each of 

 which is a portion of the ectoderm tucked in. In the bottom of 

 each sac is a specially large cell which rapidly secretes a column of 

 cuticle and builds up the chaeta. 



If we cut through the skin of an earthworm we do not make 

 our way into the cavity of the alimentary canal but 

 Anatomy! * n * * ne coeloniic cavity, in which not only the 



alimentary canal but the blood-vessels, kidneys and 

 reproductive organs apparently lie. The relation of the coelom in 

 the earthworm to the ectoderm and endoderm may be illustrated in 

 the following way. Take a piece of leaden gas-tube or of thick glass 

 tubing to represent the endodermic tube or alimentary canal, slide 

 over it a series of hollow rubber rings so that these rings are pressed 

 closely against one another in longitudinal series. Then pull a 

 cotton casing over the whole thing and so we shall have a rough 

 model of the body of an earthworm. The cotton casing will repre- 

 sent the ectoderm and the rubber rings collectively the coelom. 

 From this we learn that the coelom of the earthworm consists not 

 of one but of a series of cavities there being one ring-shaped cavity 

 in each somite, and that these cavities are separated from one 

 another by transverse walls which are called septa or dissepi- 

 ments. Each cavity has its own proper wall which is termed 



