No.1.] THE DEVELOPMENT OF BALANOGLOSSUS. 63 
was still impossible to account for later phenomena. The 
egg of hydra in the later stages of its development again 
becomes a solid mass, owing to the filling up of the interior of 
the blastocoel space by cells from the outer wall. In the case 
of hydra it is possible, but very improbable, that additional 
nutriment supplying sufficient material for the new protoplasm 
might have been absorbed from the walls of the body of the 
parent hydra. The calculation must be made in other cases 
where the same process takes place in eggs free from the 
parent. 
In the case of Tornaria we cannot make such definite state- 
ments as the animal is itself feeding (presumably) during this 
period, and the larva becomes solid (or nearly so) not by 
development of any great amount of new protoplasm, but by 
an actual decrease in the size of the larva as awhole. So great 
is this decrease that we are obliged to consider that some of the 
fluid of the blastula space is actually passed out of the animal. 
The changes of position that the digestive tract undergoes 
during this period have been already noticed. The tube itself 
shortens as its walls thicken, and in order to accommodate itself 
to the new order of things caused by the bulging out of the 
posterior plate it is forced, inasmuch as it is attached to the 
ectoderm at two points, to change its position. It is pulled 
backwards so that its most anterior point, at first far in front 
of the mouth, is now carried backwards so that the mouth 
comes to lie at the most anterior end of the digestive tract. 
The general changes in the form of the body have been 
noticed in the text. The region anterior to the horizontal 
limb of the longitudinal band becomes the proboscis. The 
horizontal limb marks the anterior edge of the collar, and the 
posterior edge of the collar appears a short distance behind in 
the region lying between the horizontal limb and the large 
circular band. All of this region is not taken up by the collar, 
for on the bulging out of the posterior plate the region in front 
of the circular band has elongated backwards. 
The circular band, that in the earlier stage formed a fringe 
around the posterior end of the larva, comes to lie around the 
middle of the elongating body of the young worm; it serves as 
