THE INVERTEBRATE BODY 



103 



space running from one end of the animal to the other, but con- 

 sists of a linear series of chambers through the center of which 

 runs the alimentary canal. The limits of these chambers are in- 

 dicated on the outside of the worm by a series of grooves which 

 encircle the body wall. In short, the body is made up of a series 

 of essentially similar units known as segments, and thus affords 

 a simple example of segmentation, which is a characteristic ex- 

 pressed in varying degrees in nearly all the higher animals. 



Many of the chief organs of the Earthworm are developed as 

 outgrowths from the walls enclosing the coelom, so that it is in 



Dorsal blood vessel 

 Chloragogen cells >^ 

 Typhlosole^^^ ,J^% 

 Intestine 



Longitudinal muscle 

 Circular muscle 

 Epidermis 

 Cuticle 



Setae / 





Coelom 

 Ventral vesse 



y Setae 



Nephridium 

 Nephrostome 



Nephridiopore 

 Nerve cord' Subneural vessel 



Fig. 61. — Transverse section through the middle region of the body of 



an Earthworm. 



this cavity that we find, for example, the main organs devoted to 

 circulation, excretion, coordination, and reproduction. Moreover, 

 the organs are symmetrically arranged with respect to the long 

 axis of the body which passes from mouth to anus. For instance, 

 the chief blood vessels and the nerve cord lie in the long axis 

 and extend from end to end, while the excretory and reproductive 

 organs are disposed in pairs on either side of this axis. Thus 

 there may be passed through the main axis a single plane which 

 divides the body into symmetrical halves, each of which is a ' mirror 

 image' of the other. The main axis, therefore, extends from the 

 mouth (anterior end) to the anus (posterior end), and the plane 



