462 



ZOOLOGY 



SECT. 



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 

 trochosphere ; 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 two families of the Polyyordiidw and the Hl^trio- 



i-'ic. 3(15. Polygordms 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 ; , ventral view of the posterior extremity ; A ,i. anus ; 

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

 Mtmr. metameree ; p. papillae ; pti-.st. peristornium ; pr.st. prostomium ; s. seta? on tentacles (/). 

 (From Parker's Biology, after Fraipont.) 



drilidce. They are all marine and all of small size. The prostomium (Fig. 365, 

 pr. st) is small, the peristomium (per. st) large. ' The segments (Mtrnr) are only 

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



