CHAETOPODA 261 



Class ARCHIANNELIDA 



Small marine annelids with simplified structure, parapodia and chaetae 

 being usually absent. 



This group was founded to receive two genera, Polygordius and 

 ProtodriluSy which were formerly considered to be primitive forms 

 from which the larger groups of annelids might be derived. From 

 time to time other genera have been included which show some, but 

 not all, of the characters which distinguish the original genera. The 

 series of diagnoses of the best known genera given below starts with 

 Polygordius and works back to forms which come very close to the 

 Chaetopoda. There can be little doubt that the Archiannelida are 

 derived from this latter group by the loss of some of its distinctive 

 features (e.g. parapodia and chaetae), and retention of juvenile 

 characters (ciliation and connection of nervous system with epidermis). 

 These changes are also found within the limits of the Polychaeta, and 

 if it was not that other characters link up its members the group might 

 well be considered as a family of polychaets. Dinophilus comes late 

 in the series because, though evidently related, it does stand rather 

 apart. It has a superficial resemblance to a small turbellarian 

 enhanced by the great reduction of the coelom. 



Polygordius (Fig. 183 B) with elongated cylindrical body, head 

 with two tentacles and ciliated pits; without parapodia or chaetae; 

 with segments of the coelom separated by septa with a pair of seg- 

 mental organs opening into each by nephrostomes ; with longitudinal 

 muscles in four quadrants, the circular muscles being usually absent; 

 with a reduced vascular system and nerve cords lying in the epidermis ; 

 with a trochosphere larva. Fig. 187. 



Protodrilus. As in Polygordius but with segmentation marked ex- 

 ternally by ciliated rings and with a longitudinal ciliated groove in the 

 middle of the ventral surface ; with a ventral muscular pharyngeal sac ; 

 hermaphrodite. 



A single species, P. chaetifer, has recently been discovered with four 

 short chaetae in each segment. 



Saccocirrus (Fig. 193 B). As in Protodrilus^ but with chaetae 

 arranged in a single bundle on each side of each segment ; with separate 

 sexes, each with complicated genital apparatus, the females with 

 spermathecae and males with a pair of protrusible penes in each 

 segment behind the oesophagus. 



Nerilla (Fig. 193 A). As in Protodrilus^ but with two bundles of 

 chaetae separated by a single cirrus on each side of each segment ; 

 three prostomial tentacles and a pair of palps ; with separate sexes and 

 a reduced number of genital segments (three in male, one in female), 



