Poly dor a nuchalis — Woodwick 
127 
broad and flangelike but certain changes in 
ciliary tracts and pigmentation associated with 
metamorphosis, to be mentioned below, have yet 
to occur. 
The vestibule leads into a long esophagus 
which joins the intestine at the anterior end of 
the sixth segment and a long rectum extends 
from about segment ten to the posterior end of 
the body. Cilia are present in the vestibule, 
esophagus, and rectum but are not indicated in 
the figure. 
The prototroch and telotroch are ordinarily 
lost shortly after settling occurs as one of the 
changes in the metamorphosis from larva to 
adult. Gastrotrochs, not mentioned previously, 
are found on segments three, five, seven, ten, 
thirteen, and fifteen. These ventral, ciliary tracts 
first appear on segment three in early stages of 
development and on succeeding segments (of 
the numbers mentioned above) as development 
continues. They are important locomotor organs 
in the short free-swimming period of P. nuchalis 
and the longer free-swimming periods of forms 
which hatch at the 3 -segment stage. 
The conspicuous dorsal melanophores are 
present only on segments twelve and thirteen 
and will be lost from these in succeeding stages. 
The pygidial melanophore and lateral pigment 
spots present on segment eight and the right 
side of segment nine are lost in metamorphosis. 
While in the egg capsule and until shortly after 
hatching, the larvae are positively phototropic; 
but at the time of settling and metamorphosis 
the melanophores and pigment spots are lost and 
the larvae become negatively phototropic. This 
suggests rather strongly a functional relation- 
ship between the phototropic change and loss 
of pigmentation. 
Two heavy spines are present in each noto- 
podium of the modified fifth segment, the larg- 
est of the body segments. Neuropodial hooded 
hooks appear first in segment seven. Branchiae 
also appear first on segment seven and in the 
13 -segment stage are partly formed on segment 
eight, as well. 
The larvae ordinarily hatch at the 12 -segment 
stage, but they may leave the egg case at the 
9-segment stage. The larvae settle, metamor- 
Plate III. 15-setiger larva (three weeks) , 0.90 mm. 
