122 



BIOLOGICAL LECTURES. 



of the component cells of the prototroch can be distinguished, 

 but in late stages only a few of them can be made out in sur- 

 face view, though tangential sections prove that they still exist. 

 As the larva elongates, the prototroch extends over more surface, 

 but the cells become thinner. The cilia are fine, not very long, 

 and are thickly and evenly distributed. In the figures they are 

 represented in a purely diagrammatic manner, being much too 

 coarse and not nearly numerous enough to reproduce the actual 

 appearance. When the larva begins to elongate, a second belt 

 of cilia, the paratroch, develops around its posterior end. The 

 paratroch consists of four ciliated cells which are derived from 

 the cell marked d"^ in Fig. 7, p. 117, and, therefore, from that 



Fig. 16. — Lateral view of Amphitrite just before the disintegration of prototroch, paratroch, 

 and mucous glands ; med. tent., median tentacle. 



quadrant which did not contribute to the secondary prototroch. 

 At first the four cells lie in a nearly straight row, but the end 

 (lateral) cells gradually approach each other, and finally meet 

 on the ventral side, as may be seen by comparing Figs. 13, 14, 

 15, and 16. Other cilia and flagella appear in tufts or bands 

 over various parts of the body, while the larva is leading a 

 free-swimming existence ; but all these, including prototroch 

 and paratroch, suddenly disappear when the larva, about five or 

 six days old, settles to the bottom. 



Figs. 13-16, when compared with the earlier drawings, illus- 

 trate the orientation of the body and the elongation of the 

 trunk. The anterior end is at the left of the picture, the pos- 

 terior end at the right ; the dorsal side at top, and the ventral 

 side at bottom. The anterior end is marked by the polar 



