338 FORMATION OF ORGANS. 



The correctness of the above interpretation is clearly shewn by the 

 metamorphosis. 



The first change consists in the pushing in of a fold of skin, between the 

 mouth and anus, towards the intestine, which at the same time rapidly 

 elongates, and forms the axis of a conical projection, which thereupon 

 becomes segmented and is thereby shewn to be the rudiment of the trunk 

 (fig. 155 B). On the elongation of the trunk in this way the prae-oral lobe 

 and its ciliated ring assume an appearance not very dissimilar to the same 

 structures in Polygordius. At the ciliated apex of the prae-oral lobe a paired 

 thickening of epiblast gives rise to the supra-cesophageal ganglia (sg). In 

 the further metamorphosis, the prae-oral lobe and its ciliated ring gradually 

 become reduced, and finally atrophy in the normal way, while the trunk 

 elongates and acquires setae. The dorsally situated processes with provisional 

 setae last for some time, but finally disappear. The young worm then 

 develops a tube and shews itself as a normal tubicolous Chaetopod. 



Formation of Organs. 



Except in the case of a few organs our knowledge of the 

 formation of the organs in the Chaetopoda is derived from 

 investigations on the Oligochaeta. 



The embryo of the Oligochaeta has a more or less spherical 

 form, but it soon elongates, and becoming segmented acquires a 

 distinct vermiform character. The ventral surface is however 

 for a considerable time markedly convex as compared to the 

 dorsal. 



The ventrally placed mouth is surrounded by a well-marked 

 lip, and in front of it is placed a small prae-oral lobe. 



The epiblast. The epiblast cells at the commencement of 

 the gastrula stage become much flattened, and on the comple- 

 tion of the invagination form an invest- 

 ment of flattened cells, only thickened in 

 the neighbourhood of the mesoblastic 

 bands (fig. 141 B and C). In the Poly- 

 chaeta at any rate the statements of 

 several investigators would seem to in- 



dicate that the cuticle is derived from the 



. FIG. 156. SECTION 



chonon. It is difficult to accept this THROUGH THE HEAD OF A 



conclusion, but it deserves further in- (Ate 



VCStigation. Kleinenberg.) 



NervOUS system. The most im- C -S- cephalic ganglion; 



, .. cc. cephalic portion of the 



portant organ derived from the epiblast body cavity ; x. oesophagus. 



