PHYLUM ANNULATA 



491 



CLASS III. ARCHI-ANNELIDA. 



More primitive in some respects than the other Annulata are the Archi- 

 Annelida, comprising only the family Polyyordiidcp. They are marine worms 

 with narrow, elongated, cylindrical body. The prostomium (Fig. 406, Pr. st) 

 is small, the peristomium (Per. st) large. The segments (Mtmr) are only 

 faintly marked off externally for the most part, though the internal division 

 of the ccelome by means of septa is complete. Parapodia and setse are 

 absent, but the prostomium bears a pair of tentacles (t). Several pairs of 

 simple nephridia are present. The position of the nervous system (Fig. 408) 

 is more primitive than in the Annulata in general ; it is continuous with 



fr.st 



FIG. 406. Polygordius neapolitanus. A, the living animal, dorsal aspect, about five times 

 natural size ; B, anterior end, lateral view ; C, ventral view of the same ; D, portion of the 

 body showing the metameres ; E, ventral view of the posterior extremity ; An. anus ; 

 An. seg. anal segment ; c. p. ciliated pit ; yr. grooves between metameres ; Mth. mouth ; 

 Mtmr. metameres ; p. papillae ; Per. st. peristomium ; Pr. st. prostomium ; s. papillae on 

 tentacles (t). (From Parker's Biology, after Fraipont.) 



the epidermis, and not separated from it by mesodermal elements as in most 

 of the others. A pair of ciliated grooves (c. p.) are probably to be looked 

 upon as organs of special sense. 



The family Polygordiidce includes two genera Polygordius and Protodrilus. 

 There are a pair of prostomial tentacles, long in Protodrilus, short in Polygor- 

 ilius, and a pair of ciliated pits. The segmentation is only very indistinctly 

 marked externally in Protodrilus by circlets of cilia ; in Polygordius it is 

 indistinct in front, but better marked behind. In Polygordius lacteus a 



