The atrochæ contain forms ([fig. 139]) in which the larva is at first coated by an uniform covering of cilia, which, though it may subsequently disappear from certain areas, does not break up into a series of definite bands.
The monotrochæ or cephalotrochæ are larvæ in which only a single præ-oral ring is developed ([fig. 150] B).
Fig. 150. Two Chætopod larvæ. (From Gegenbaur.)
o. mouth; i. intestine; a. anus; v. præ-oral ciliated band; w. peri-anal ciliated band.
In the telotrochæ there is present a præ-oral and a post-oral, i.e. peri-anal ring ([fig. 150] A); the latter sometimes having the form of a peri-anal patch.
The polytrochæ are segmented larvæ with perfect or imperfect rings of cilia on the segments of the body—usually one ring to each segment—between the two characteristic telotrochal rings. When these rings are complete the larvæ are polytrochæ proper, when they are only half rings they are either nototrochæ or gasterotrochæ. Sometimes there are both dorsal and ventral half rings which do not however correspond, such forms constitute the amphitrochæ.
In the mesotrochæ one or two rings are present in the middle of the body, and the characteristic telotrochal rings are absent. Larvæ do not necessarily continue to belong to the same group at all ages. A larva may commence as a monotrochal form and then become telotrochal and from this pass into a polytrochal condition, etc.
The atrochal forms are to be regarded as larvæ which never pass beyond the primitive stage of uniform ciliation, which in other instances may precede that of definite rings. They usually lose their cilia early, as in the cases of Serpula and other larvæ described below.