124 



FOUNDATIONS OF BIOLOGY 



other. The main axis, therefore, extends from the mouth 

 (ANTERIOR END) to the anus (POSTERIOR END) , and the plane 

 which divides the body into right and left sides passes through 

 the upper (DORSAL) and lower (VENTRAL) side. This general 



dors, v 



typh 



neph 



hep 



FIG. 68. Transverse section through the middle region of the body of the Earth- 

 worm, circ. mus, circular muscle fibers; coel, coelom; cut, cuticle; dors, v, dorsal blood 

 vessel; epid, epidermis; ext. neph, external opening of nephridium; hep, gland cells; 

 long, mus, longitudinal muscles; neph, nephridium; nephrost, internal opening of ne- 

 phridium; n. co, nerve cord; set, setae; sub. n. vess, subneural vessel; typh, typhlosole; 

 cent, v, ventral vessel. (From Parker and Haswell, after Marshall and Hurst.) 



disposition of organs is known as BILATERAL SYMMETRY and 

 is characteristic of all higher animals. 



The body of the Earthworm is radically different from that 

 of Hydra, exhibiting as it does such essential features as 

 coelom, bilateral symmetry, and metamerism, which are 

 adopted by higher animals as the basic plan of organization. 



