DEVELOPMENT OF THE EARTHWORM. 177 



originate, according to Kleinenberg, independently of the 

 ventral cord from a median unpaired apical plate of ecto- 

 derm, while according to Wilson they arise along with the 

 ventral cord, and have their foundations in the thickened 

 anterior end of each of the two neural rows. 



The history of the excretory system is complex, (a) At 

 the anterior end of young embryos, a group of ectoderm 

 cells, dorsal in position, forms a larval excretory organ, which 

 wholly disappears in later stages, {b) Next appear two 

 ciliated canals in the anterior region, closed internally, 

 opening on the head. These are known as "provisional 

 nephridia" or "head-kidneys.'' They degenerate as the 

 permanent excretory organs develop, ic) The numerous 

 permanent nephridia are for the most part ectodermic, 

 arising from the rows of nephridial cells already described. 

 Two parts of each nephridium, however, have a mesoblastic 

 origin, viz., the innermost part — the ciliated funnel, and 

 the peritoneal investment which ensheathes the whole 

 organ. 



By the invagination of the blastosphere, a globular gas- 

 trula cavity is formed. This forms the archenteron, — the 

 future mid-gut, — and elongates with the growth of the embryo. 

 To the completion of the entire alimentary canal, however, 

 two other processes are necessary, an in-tucking of ectoderm 

 from in front — the stomatodaum — which pushes the archen- 

 teron backwards and forms the future pharynx, and a similar 

 in-tucking of ectoderm from behind — \h& prociodczum — which 

 meets and fuses with the archenteron, and forms the anus 

 and a small portion of the posterior gut. As this com- 

 pound formation of the gut is general among animals, we 

 may restate it in a tabular form : — 



or Gut. 



1: 



''Anterior ectodermic invagination — the 

 stomatodaeum or " fore-gut." 

 ... „ . , Median gastrula cavity lined by endo- 



Alimentary Canal J derm— the archenteron, mesenteron, 

 or " mid-gut." 

 Posterior ectodermic. invagination — 

 the proctodseum or "hind-gut." 



So far, then, the ectoderm primarily forms the epidermis, 



