280 



FORMATION OF ORGANS. 



The correctness of tbe 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 prse-oral 

 lobe and its ciliated ring assume an appearance not very dissimilar to 

 the same structures in Polygordius. At the ciliated apex of the prse-oral 

 lobe a paired thickening of epiblast gives rise to the supra-cesophageal 

 ganglia (sy). In the further metamorphosis, the prse-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 setje last for some time, but finally disappear. 

 The young worm then develops a tube and shews itself as a normal 

 tubicolous Chsetopod. 



Formation of Organs. 



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

 tion of the organs in the Chaetopoda is derived from investigations on 

 the Oligochseta. 



The embryo of the Oligochseta 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 consider- 

 able 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 completion of the 

 invagination form an investment of flattened cells, only thickened 

 in the neighbourhood of the mesoblastic bands (fig. 141 B and C). 

 In the Polychoeta at any rate the statements of several investigators 

 would seem to indicate that the cuticle is derived from the chorion. 



It is difficult to accept this con- 

 clusion, but it deserves further in- 



cc~ 



vestigation. 



FIG. 15fi. SECTION THROUGH THE 



HEAD OF A YOUNG EMBKYO OF LuM- 

 BRICUS TRAPEZOIDES. 



enberg. ) 



e.g. cephalic ganglion; cc. ce- 

 phalic portion of the body cavity; 

 x. oesophagus. 



Nervous system. The most im- 

 portant organ derived from the epi- 

 blast is the nervous system ; the origin 

 of which from this layer was first es- 

 tablished by Kowalevsky (No. 342). 



It arises 1 (Kleinenberg, No. 341) 

 from two at first quite distinct struc- 

 (After Klein- tures, viz. (1) the supra-cesophageal 

 rudiment and (2) the rudiment of the 

 ventral cord. The former of these 

 takes its origin as an unpaired dorsal 



1 For further details, vide general chapter on Nervous System. 



