CHAPTER VII 



ANNELIDA 



Classification adopted 



Archiannelida 



Chaetopoda 



Polychaeta 



fNereidiformia 

 Spioniformia 

 Terebelliformia 

 Capitelliformia 

 Scoleciformia 

 Scabelliformia 



.Hermelliformia 



.Oligochaeta 



fAcanthobdellidae 

 Hirudinea - Rhyncobdellidae 

 [ Gnathobdellidae 



THE group of segmented worms known as Annelida has furnished 

 subjects for an immense amount of embryological study, but 

 there are a great many points in their development still unsettled 

 which offer a wide field for future research. Although widely diverse 

 from each other in their adult structure the members of the group 

 show a remarkable uniformity in their early development, so that 

 the complete description of a single type will serve as a guide to 

 what is known about the development of all. 



Annelida are divided into Archiannelida, including Polygordius 

 and a few allied forms which never develop chaetae and are devoid 

 of external circular muscles ; Polychaeta, the central group, including 

 worms with numerous chaetae, well-developed parapodia, and external 

 circular muscles ; Oligochaeta, freshwater and terrestrial worms, with 

 few chaetae, complicated genital organs, no parapodia, but provided 

 with external circular muscles ; and Hirudinea, extremely modified 

 forms with obscure segmentation, no chaetae or parapodia, but with 

 external circular muscles, extremely complicated genital organs, and 

 suckers used for progression. 



( )f these forms the most primitive, and the one which shows the 

 longest larval development, is the Archiannelidan Polygordius. The 



128 



