520 Howard Edwin Enders. 



where the ciliated ring is interrupted, are two slight mesial thicken- 

 ings that represent the ventral adhesive disc of the twelfth segment. 

 A pair of similar thickenings on each successive segment of this 

 region indicates the beginnings either of adhesive discs or of neuro- 

 podia. 



The thirteenth segment is represented in the larva by the hindmost, 

 or third incomplete ciliary ring. On the ventral side is the rudi- 

 ment of the adhesive disc as mentioned above, and in a mesial posi- 

 tion directly back of the dorsal side of the ciliated ring is a bilobed 

 tegumentary outgrowth (Fig. 13) that represents the accessory feed- 

 ing organ of the thirteenth segment of a transformed larva. Its 

 internal ciliation is possibly derived from a part of the ciliated ring. 



The fourteenth, fifteenth and sixteenth segments are somewhat 

 thickened, saucer-shaped discs that decrease in size regularly back- 

 wards, and have their convex sides turned fonvard. They develop 

 into the palettes or "fans," in the transformed worm. Small muscle 

 cells are present in the walls of these segments. They also form 

 cross-strands that unite the forward wall to the rear w^all, as is seen 

 in the palettes of the worms. They are not in rhythmic motion in 

 this stage, but are often telescoped into one another by the axial 

 contraction of the larva. The mesial thickening on the ventral side 

 of each segment has been mentioned : the first becomes the small 

 adhesive disc of the first palette, and the others expand into the short, 

 thick neuropodia of the second and third palettes. 



Posterior region. — The posterior region is still very small (Figs. 

 11-13). It consists of two sexual segments, each of which bears a 

 pair of divergent club-shaped to spatulate appendages that extend 

 obliquely backward, and the rudiments of several others, in the form 

 of buds, that are ventral to the anus. Each of the larger divergent 

 appendages encloses a single straight seta of the same form as those 

 found within the conical notopodia of the sexual segment of the 

 adult worm. That these are really the notopodia of the segment, 

 and are formed ventral to the anus but later migrate lateralwards and 

 finally point dorsalwards, is clearly shown in a large worm that was 

 undergoing regeneration of the segments of this region (Fig. 14). 

 In this specimen there were nine segments in various stages of devel- 



