CHsETOPODA. 



279 



absent in the adults of these 'forms. The Seclentaria thus pass 

 through a larval stage in which they resemble the Errantia. 



Paired auditory vesicles of a provisional character have been 

 found on the ventral side of the body, in the fourth segment behind 

 the mouth, in the larva of Terebella conchilega (Claparede). 



Mitraria. A peculiar larval Chretopod form known as Mitraria, the 

 metamorphosis of which was first worked out by Metsclmikoff, deserves 

 a special notice. 



This form (fig. 155 A) in spite of its remarkable appearance can easily 

 be reduced to the normal type of larva. 



The mouth (m) and anus (an) (fig. 155 A) are closely approximated, and 

 situated within a vestibule the edge of which is lined by a simple or lobed 

 ciliated ring. The shape of the body is somewhat conical. The cavity 



A 



FIG. 155. Two STAGES IN THE DEVELOPMENT OF MITRARIA. (After Metsclmikoff. ) 



m. mouth; an. anus; sr/. supra-oesophageal ganglion; br. provisional bristles; 

 pr.b. prte-oral ciliated baud. 



of the vestibule forms the base of the coue, and at the apex is placed a 

 ciliated patch (</). A pair of lobes (l>r) bear provisional setae. The 

 alimentary canal is formed of the three normal parts, (.esophagus, stomach, 

 and intestine. 



To compare this larva with an ordinary Chsetopod larva one must 

 suppose that the alimentary canal is abnormally bent, so that the post-oral 

 ventral surface is reduced to the small space between the mouth and the 

 anus. The ciliated band surrounding the vestibule is merely the usual 

 pne-oral band, borne on the very much extended edge of the prse-oral lobe. 

 The apex of the larva is the front end of the prse-oral lobe with the 

 usual ciliated patch. The two lobes with provisional bristles are really 

 dorsal and not posterior. 



