PHYLUM ANSULATA 



503 



result is attained in Phoronis do not occur. Again, while in Phoronis 

 the tentacles of the adult may be looked upon as formed by the 

 development of processes along the line occupied by the post-oral 

 circlet of cilia, in the Sipunculida the tentacular lobes have 

 nothing to do with the post-oral circlet, but are formed by the 

 growth of a series of lobes from the margin of the mouth itself. 

 The larva of the Sipunculida again is, as already pointed out, very 

 nearly related to the larva of the Chsetopoda, and is a typical 

 trochophore ; while the Actinotrocha larva of Phoronis diverges 

 somewhat widely from that type. 



CLASS III. ARCHI-ANNELIDA. 



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

 Annelida, comprising only the family Pdygordiidce to which may perhaps be added 



fr.st 



Fio. 300. 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 rnetameres ; E, ventral view of the posterior extremity ; An. anus ; 

 An" sert. anal segment ; c. p. ciliated pit ; gr. grooves between metameres ; Mth. mouth; 

 Mt,,,-,:' metameres; p. papillae; per.st. peristomium; pr.st. prostomium ; s. papilla? on 

 tentacles (0- (From Parker's Biolotiy, after Fraipont.) 



Ctenodnhis. They are marine worms with narrow, elongated, cylindrical body. 

 The prostomium (Fig. 399, pr. *t) is small, the peristomium (per. st) large. The 

 segments (Mlmr) are only faintly marked off externally for the most part, though 



