GENERAL PLAN OF THE BODY. 



47 



Plan of the Body. The body of the earthworm (Fig. 23), 

 like that of all higher animals, consists of two tubes, one (al\ 

 within the other and separated from it by a considerable space 

 or cavity (cce). The inner tube is the alimentary canal, open- 

 ing in front by the mouth and behind by the anus ; the outer 

 tube is the body-wall, and its cavity is the body-cavity or ccelom. 



FlG. 23. A, diagram of the earthworm as seen in a longitudinal section of the body, 

 showing the two tubes, the ccelom, and the dissepiments. B, diagram of cross- 

 section : nl, alimentary tube ; OH, anus ; c<, ccelom ; m, mouth. C, diagram 

 showing the arrangement of some of the principal organs : m, mouth ; a, anus ; 

 a?, alimentary canal; d#, dissepiments; d.r., dorsal blood-vessel; r, ventral or 

 sub-intestinal vessel ; c.r., circular vesselb; n, nephridia or excretary organs; e.g., 

 cerebral ganglia ; t\0., ventral chain of ganglia ; o.d., oviduct; o.rf., ovary. The 

 arrows indicate the course of the circulation of the blood. 



The ccelom is not, however, a free continuous space extending 

 from end to end, but is divided transversely by a series of thin 

 muscular partitions, the dissepiments, into a series of nearly 

 closed chambers traversed by the alimentary canal. Each com- 

 partment corresponds to one somite, the dissepiments being 

 opposite the external furrows mentioned on p. 45. All the 

 organs of the body are originally developed from the walls of 

 these chambers, and some of them (e.g., the organs of excretion) 

 project into the cavities of the chambers, that is into the co3lom. 



