Their mode of development is well illustrated by the case of Terebella nebulosa (_vide_ Milne-Edwards, No. 347). The embryo is at first a nearly spherical ciliated mass. One end slightly elongates and becomes free from cilia, and, acquiring dorsally two eye-spots, constitutes a præ-oral lobe. The elongation continues at the opposite end, and near this is formed a narrow area free from cilia. The larva now has the same characters as the atrochal Eunicidan larva described above. It consists of a non-ciliated præ-oral lobe, followed by a wide ciliated band, behind which is a ring-like area free from cilia; and behind this again a peri-anal patch of cilia. The ring-like area free from cilia is, as in the Eunicidan larva, the region which becomes segmented. It soon becomes longer, and is then divided into two segments; a third and fourth etc. non-ciliated segment becomes successively interposed immediately in front of the peri-anal patch; and, after a certain number of segments have become formed, there appear on some of the hinder of them short tubercles, provided with single setæ (the notopodia), which are formed from before backwards, like the segments. The mouth, anus, and intestine become in the meantime clearly visible. The mouth is on the posterior side of the ciliated band, and the anus in the centre of the peri-anal patch. The ciliated band in front now becomes contracted and provided with long cilia. It passes below completely in front of the mouth, and constitutes, in fact, a well-marked præ-oral ring, while the cilia behind constitute an equally marked peri-anal ring. The larva has in fact now acquired all the characters of a true telotrochal form. Only a comparatively small number of Chætopod larvæ remain permanently telotrochal. Of these Terebella nebulosa, already cited (though not Terebella conchilega), is one; Polygordius, Saccocirrus and Capitella are other examples of the same, though in the latter form the whole ventral surface becomes ciliated. The majority of the originally telotrochal forms become polytrochal. In most cases the ciliated rings or half rings of the polytrochal forms are placed at equal distances, one for each segment. They are especially prominent in surface-swimming larvæ, and are in rare cases preserved in the adult. In some instances (_e.g._ Nerine and Spio) the ventral half rings, instead of being segmentally arranged, are somewhat irregularly distributed amongst the segments, so that there does not seem to be a necessary correspondence between the ciliated rings and the segments. This is further shewn by the fact that the ciliated rings are not precursors of the true segmentation, but are developed after the establishment of the segments, and thus seem rather to be secondarily adapted to the segments than primarily indicative of them. In most Polytrochæ the rings are incomplete, so that they fall under the category of Nototrochæ or Gasterotrochæ.