CHAPTER VII 



ANNELIDA 



Classification adopted 



Archiannelida 



Chaetopoda 



Polychaeta 



Nereidiformia 



Spioulformia 



Terebelliformia 



Capitelliformia 



Scoleciformia 



Scabelliformia 



.Hermelliformia 



Oligochaeta 



(Acanthobdellidae 

 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. 



Of these forms the most primitive, and the one which shows the 

 longest larval development, is the Archiannelidan Polygordius. The 



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