With reference to the sense organs it may be noted that the eyes, or at any rate the cephalic pigment spots, are generally more numerous in the embryo than in the adult, and that they are usually present in the larvæ of the Sedentaria, though absent in the adults of these forms. The Sedentaria 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 (Claparède). Mitraria. A peculiar larval Chætopod form known as Mitraria, the metamorphosis of which was first worked out by Metschnikoff, 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 of the vestibule forms the base of the cone, and at the apex is placed a ciliated patch (_sg_). A pair of lobes (_br_) bear provisional setæ. The alimentary canal is formed of the three normal parts, oesophagus, stomach, and intestine. [FIG. 155. TWO STAGES IN THE DEVELOPMENT OF MITRARIA. (After Metschnikoff.) _m._ mouth; _an._ anus; _sg._ supra-oesophageal ganglion; _br._ provisional bristles; _pr.b._ præ-oral ciliated band.] To compare this larva with an ordinary Chætopod 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 præ-oral band, borne on the very much extended edge of the præ-oral lobe. The apex of the larva is the front end of the præ-oral lobe with the usual ciliated patch. The two lobes with provisional bristles are really dorsal and not posterior. The correctness of the above interpretation is clearly shewn by the metamorphosis.