8 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The mouth opens on the ventral surface, and a dorsal and ventral pair 
of “teeth” have made their appearance in the cesophagus (see Figure 
20). The worms are now about 0.8 mm. in length. Internal as 
well as external evidences of segmentation now appear (see Figure 8, 
Plate 1) and the dissepiments (ds) are complete. The walls of the 
mid gut are very thick and consist of large, irregularly shaped, highly 
vacuolated cells containing a number of yolk spheres. The cells of 
the cesophagus (oes) are of an epithelial character. The peripheral 
circular muscles and the deeper lying longitudinal muscle strands are 
beginning to appear, and the ventral nerve chain () is very apparent- 
In fact, the animal is no longer a larva, but is a young worm. 
Figures 21-23, 25-27, illustrate the condition of the worm at the 
end of the 26th day. There are now five pairs of parapodia, and the 
dorsal and ventral cirri of the posterior segment have become long and 
prominent (see Figure 23). The dorsal and ventral jaws of the cesoph- 
agus are shown in the side view of the head given in Figure 22. 
Figures 26 and 27 are views from above and from the side, respectively, 
of the dorsal pair of jaws. The condition of the ventral pair of jaws 
is still quite similar to that in the 16-day-old worm shown in Figure 
20. The worms are now 1.2 mm. long. They burrow readily 
beneath the surface of sand, but never swim through the water. 
Figure 24 shows the condition of a worm 34 days old. The animal 
is now 1.5 mm. in length, and there are still only five pairs of para- 
podia. The mature coloration is beginning to appear in two reddish- 
colored spots immediately back of the eyes. I did not succeed in 
rearing any worms beyond this condition, and know nothing of the 
mode of formation of the prestomium and cephalic cirri of the adult 
worm. It will be observed that in the young worm the mouth opens 
on the ventral surface and the prazestomium is supra-oral, while in the 
adult worm the preestomium and cephalic cirri are sub-oral (compare 
Figures 3 and 22). 
General Conclusions. 
Remarkably little has been written concerning the egg-laying habits 
of Polychetae. Wilson (’92, p. 371) states that the eggs of Nereis 
limbata and N. megalops are discharged at night while the animals 
are swimming upon the surface of the water. The egg-laying season 
extends at least from June until September. “The animals appear in 
abundance only on warm still nights, and even then are rarely found 
