122 



FOUNDATIONS OF BIOLOGY 



Many of the chief organs of the Earthworm are developed 

 as outgrowths from the walls enclosing the coelom, so that 

 it is in this cavity that we find, for example, the main parts 

 of the organ systems devoted to circulation, excretion, and 

 reproduction, as well as the nervous system. Moreover, the 



71 



FIG. 66. Diagrams of the body plan of the Earthworm. A and C, longitudinal 

 sections; B, transverse section, a, aortic loops of the blood vascular system; al, ali- 

 mentary canal; an, anus; e.g., brain (cerebral ganglion) ; coe, coelom; cv, blood vessels 

 (parietal) to body wall; ds, partitions (septa) between the segments; d.v., dorsal blood 

 vessel; ra, mouth; n, nephridia; o, ovary; o.d., oviduct; s.i., ventral blood vessel. 

 {From Sedgwick and Wilson.) 



organs are symmetrically disposed 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 organs of the 

 excretory and reproductive systems 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 sym- 

 metrical halves, each of which is a 'mirror picture' of the 



