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.
The larva of Odontosyllis is an example of the former, and that of Magelona of the latter. The larvæ 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 Claparède and Metschnikoff, develops a complete ciliated ring on each segment: and the præ-oral ring, though at first single, becomes at a later period divided into two. This form is further exceptional in that the ciliated rings are persistent in the adult.
The unimportance of the character of the rings in the polytrochal forms is shewn by such facts as the absence of these rings in Terebella nebulosa and the presence of dorsal half rings in Terebella conchilega.
The mesotrochal forms are the rarest of Chætopod larvæ, and would seem to be confined to the Chætopteridæ.
Their most striking character is the presence of one or two complete ciliated rings which girth the body between the mouth and anus. The whole body is further covered with short cilia. The anus has a distinct dorsal situation, while on its ventral side there projects backwards a peculiar papilla.
The total absence of the typical præ-oral and of the peri-anal bands separates the mesotrochal larvæ very sharply from all the previous types.
A characteristic of many Chætopod larvæ is the presence of a bunch of cilia or a single flagellum at the apex of the præ-oral lobe. The presence of such a structure is characteristic of the larval forms of many other groups, Turbellarians, Nemertines, Molluscs, etc.