280 



EMBRYOLOGY 



skin, which grows out backwards, and surrounds like a mantle the part 



of the body lying behind the head. 



The first two segments lying behind the head are conspicuous in many 



Polychaeta {Terebella, Oph- 

 ryutrocha) by the fact that 

 they are destitute of seg- 

 mental appendages ; this 

 fact has caused them to be 

 reckoned as belonging to the 

 head, which is thus sup- 

 posed to arise from several 

 segments. However, the 

 manner of formation of 

 their internal organs (neural 

 and mesodermal), which are 

 begun like those of the body 

 segments, is an argument 

 against this (Salensky). 

 Differences of opinion exist 

 among the different investi- 

 gators concerning the origin 

 of the head itself, for some 

 of these maintain that it is 

 formed from the preoral por- 

 tion of the Trochophore 

 alone ; while, according to 

 others, post-oral parts of the 

 larva also enter into its for- 

 mation. 



In forms which, like 

 Exogone gevimifera and 

 Gruhea limbata, brood 

 their eggs, the stage of 

 the free larva may be 

 altogether omitted, the 

 embryo breaking 

 through the eg^ en- 

 velope in the form of 

 the young worm al- 

 ready provided with a 

 number of segments, 

 parapodia, and cirri (Viguier). It is these conditions which 

 recall those in the Oligochaeta, more especially since one of 



Fig. 128. — A, B, larval stages of Psygmohran- 

 chus protensus (after Salensky). ^.Trochophore 

 with pre- and post-oral ciliated bands and adoral 

 ciliated groove (seen from the ventral surface); 

 m, region of the subsequently formed mouth. 

 opening ; ent, still undifferentiated entodermal 

 mass within; B, later stage with gills (k); b, 

 fundaments of the setas ; fcr, collar ; and vd, md, 

 ed, fore-, mid-, and hind-gut. 



