CH.ETOPODA. 275 



cilia, and, acquiring dorsal ly two eye-spots, constitutes a prse-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 

 prse-oral lobe, followed by a wide cib'ated 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 succes- 

 sively interposed immediately in front of the perianal 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 setse (the noto- 

 podia), 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 perianal 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 prse-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 Chsetopod larvae 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 poly- 

 trochal. 



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 larvae, 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 segmenta- 

 tion, 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 Polytrochae the rings are incomplete, so that they fall 

 under the category of Nototrochae or Gasterotrochae. 



The larva of Odontosyllis is an example of the former', and that of 

 Magelona of the latter. The larva? of Nerine and Spio, already quoted 

 as examples of an unsegmented arrangement of the ventral ciliated half 

 rings, are both amphitrochal forms. 



As an example of a polytrochal form with complete ciliated rings 

 Ophryotrocha puerilis may be cited. This form, discovered by Claparede 

 and Metschnikoff, develops a complete ciliated ring on each segment : 



182 



