CH^TOPODA. 



193 



either side (nephridioblasts) form parts of the nephridia (Fig. 84 (4)), 

 while of the fourth row nothing definite is known. Each row, ending 

 behind in a single cell, widens out and deepens as it is traced forwaids. 

 The neural and mesoblastic rows can be traced round the mouth, and 

 help to form the prostomium ; the others fade away at the sides of the 

 stomodaeum. 



Let us sum up this complex history : — 



Fertilised 

 ovum. 



Blastosphere 



or 



blastula. 



Two-layered 



gastrula 



with primitive 



mesoblasts. 



Epiblast 



or 

 ectoderm 



(a) The original outer layer 

 becomes the epidermis. 



(£)The secondary inner strat- 

 um consists of neuroblasts 

 which form the nervous 

 system, of nephridioblasts 

 which form parts of the 

 nephridia, and of lateral 

 cells of unknown function. 



Mesoblast 



mesoderm f M" 31 ^ , 

 formed from 1 J" ood vessels ; , ... 

 the division of i nner ? arts of " e P h "d>a. 



the primitive 

 "mesoblasts." 



I Reproductive organs. 



Hypoblast fLiningof 

 V endoderm. \"i>d-gut. 



General development of the organs. — The origin of the more im- 

 portant organs may be briefly noticed. 



In the nervous system, while the ventral cord arises from the neuro- 

 blasts, the two cerebral ganglia originate, according to Kleinenberg, 

 independently from a median unpaired apical plate of ectoderm, 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. 

 (6) Next appear two ciliated canals in the anterior region, closed inter- 

 nally, but opening externally on the head. These are known as 

 " provisional nephridia " or "head kidneys." They degenerate as the 

 permanent excretory organs develop. (<r) 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 or the 

 ciliated funnel, and the peritoneal investment, which ensheaths the 

 whole organ. 



The gastrula cavity 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 intucking of ectoderm from in front — the stomodaum or ' 'fore-gut " 

 — which pushes the archenteron backwards and forms the future 

 pharynx, and a similar intucking of ectoderm from behind — the 



13 



