22 MORGAN. iVor. Is 
up the interior of the central cavity of each slit. These bars, 
or tongue-bars, hang freely into the opening of the gill-slits 
and are formed by an ingrowth of the dorsal wall. Beneath 
the gill-slits the oesophagus is broad from side to side, and at 
its end a constriction is found. Behind this the digestive 
tract is a small tube, opening at the posterior end of the body. 
In the region of the circular band the digestive tract is swollen 
out and filled with a collection of sand. 
On account of the greater opacity of the larva none of the 
changes taking place inside of the proboscis, e¢c., could be 
made out. 
The young worm shown in the next figure (Fig. 9, Pl. II) 
was caught on June 24 and the drawing made on July 1, — 
it had been in the aquarium about six days. The principal 
change is in size, but the relative proportions of the body have 
not materially changed. The proboscis is relatively shorter, 
and the collar both larger and broader. 
The body behind the collar has not only elongated to nearly 
double its former length, but has enlarged correspondingly in 
other dimensions. The circular band is still marked by a 
circle of pigment, and is somewhat more irregular than in the 
preceding stage. It will be seen that the body has elongated 
both before and behind the band. In the present figure the 
part of the body behind the band is the longer, but this is due 
to the relative amount of contraction of the parts, for in later 
stages the reverse conditions were often found. 
Pigment flecks cover the body and are more widely separated 
than in the preceding stage. Whether the pigment spots found 
in the ectoderm at this stage are the direct descendents of those 
of an earlier stage I am unable to say. The pigment spots 
put into the first drawings were not drawn with sufficient ac- 
curacy nor do my notes do more than point to the relative 
abundance in the succeeding stages. “I regret this the more 
for they might have furnished additional data as to the method 
of growth. 
Turning to the gill-region we find the conditions much the 
same as in the preceding stage. Four pairs of gill-slits open 
