55 



position of this somewhat obscure group, and when fuller 

 knowledge as to its affinities has been acquired, it may turn 

 out that the Discophora are really allied, not to the higher 

 worms, but to the Platyelmia."*' 



From the ancestral Chaetopoda, which compose this 

 upper part of the vermean axis, the Oligochseta probably 

 diverged in one direction, and the small group Achgeta 

 (Polygordius) in another. The various larval stages in the 

 development of Polygordius show how the metameric seg- 

 mentation, which is found in all the Chaetopoda, was acquired 

 in their ancestral history. Beginning with the Trochosphere 

 stage (fig. 18), successive segments are formed at the post- 



Fig. 18. Trochosphere. p, prse-oral lobe ; q, post-oral or abdominal part of 

 body ; c, prse-oral circle of large cilia ; 2^ c, perianal circle of ciUa ; m, mouth ; 

 oe, oesophagus (stomodseimi) ; s, stomach ; i, intestine; a, amis; g, supra-oesophageal 

 ganglion ; o, eye-spot. 



erior end of the body, gradually converting it into an elon- 

 gated worm -like form, while the greater part of the original 

 Trochosphere remains as the head region of the adult worm. 

 In the Oligochaeta and the Achaeta no appendages are present, 

 although in the former there are chitinous bristles (setae) 

 which project from the body wall. The primitive Oligochaeta 

 have diverged into several small groups, all of which are 

 hermaphrodite, and possess highly specialised reproductive 

 organs,. more or less resembling those of Lumbricus. 



* See Bourne, Quart. Journ. Mlc. Sc, vol. xxiv, No. xcv, p. 493 (1884). 



