3i6 PERIPATUS, MYRIOPODS, AND INSECTS. 



primitive enteron (or gut), or as a space or spaces in the unsegmented 

 or segmented mesoderm. Now, in Arthropods the apparent body cavity 



Fig. I02. — Diagrammatic cross section of an Inverte- 

 brate, with a primary body cavity {be) which is shaded. 

 (After ZiEGLER. ) 



cc, Ectoderm; bl.^ bladder of nephridium (as in Cnistaceans) ; ex.., 

 excretory duct ; gn.., genital organ ; n., ventral nerve cord ; g.^ gut: yC, 

 funnels of nephridia (as in worms). 



of the adult is not a true ccelome, it consists of a set of secondarily 

 derived vascular spaces ; it has been called a pseudocoele or a hjemoccele. 



Fig. 103. — Diagrammatic cross section of an Inverte- 

 brate, with a secondary and a primary body cavity. (After 



ZlEGLER. ) 



cc. Ectoderm; s.bc, secondarj> body cavity (as in Lamellibranchs); 

 g., gut ; h.c, primary body cavity (shaded); c.x-., excretory aperture of 

 nephridium ; gn., genital organ ; «., ventral nerve ganglia. 



The true ccelome of Arthropods is very much restricted in the adult, all 

 the more so that most Arthropods (e.g., Insects) have no distinct 

 nephridia. 



