GENERAL PLAN OF TUE BODY. 



47 



Plan of the Body. The Ludy of the earthworm (Fi(r. 23), 

 like tliat of all higher aniniaLs, c(jiisists of two tubes, one (al) 

 within the other and separated from it by a considerable space 

 or cavity {coe). The inner tube is the allment'ir;/ canal,, open- 

 ing in front by tlie mouth and l^ehind by the anuH ', the outer 

 tube is the body- wall, and its cavity is the hodtj-cavity or Ciduni, 



an 



c.vr ^rf. 



■^^^.m^^ 



0. c.cL. n ^■^^ 



JZ 



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

 showing the two tubes, the coelom, and the dissepiments. JB, diagram of cross- 

 section : (7/, alimentary tube; an, anus; Cfr, ccelom; »i, mouth. C, diagram 

 showing the arrangement of some of the principal orgaus : //i, moutli ; an, anus ; 

 al, alimentary canal; ds, dissepiments; d.r., dorsal blood-vessel; r, ventral or 

 sub-intestinal vessel ; c.r., circular vesselb ; ?j, nephridia or excretary organs; <•.(/., 

 cerebral ganglia ; r.(/., ventral chain of ganglia; r>.(/., oviduct; o.d., ovary. The 

 arrows indicate the course of the circulation of the blood. 



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

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

 muscular partitions, the dlssejnments, into a series of nearlv 

 closed chambers traversed by the alimentary canal, Eacli (m.iu- 

 partment corresponds to one somite, the dissepiments ])eing 

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

 organs of the body are originally developed from tlie walls of 

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

 project into the cavities of the cliambers, that is into the cadom. 



