No. 1.] _THE DEVELOPMENT OF BALANOGLOSSUS. ¥ | 
of the different parts. Subsequently, some details were added 
to the figures from the preserved animals. 
The series of figures given to illustrate the development is, 
of course, arbitrary inasmuch as the series of changes is a con- 
tinuous one, but it is convenient to be able to refer briefly to 
the figures as indicating definite stages of development. 
The description is divided into two parts. In the first 
division the changes to be seen from the exterior are dealt 
with. The observations will be checked by information 
gathered from serial sections. 
The second division deals more particularly with the organ- 
ography and histology of the larva as gathered from surface 
views, dissections and serial sections. 
The smallest and youngest larva obtained is shown in Fig. 1, 
Pl. I. It measured 1144 mm. in length, and a little less in 
cross diameter. The larva is irregularly pentagonal in side 
view as seen in the figure. The apex of the larva, uppermost 
in the figure, is the anterior end. The lower (down in the 
figure) is the posterior end. The side where the mouth opens, 
to the left in the figure, is ventral and the opposite side, to the 
right in the figure, is dorsal (the nervous system subsequently 
develops on this side). Left and right are, therefore, the sides 
towards and away from the observer. The larva is very trans- 
parent and the tubular digestive tract lying in the middle of 
the body is distinctly visible from the exterior. At the 
anterior end are two small oval pigment spots, the so-called 
eyes ; they are not seen in the figure. 
Around the posterior end of the larva is a band of large 
cilia — the posterior or circular ciliated band. The posterior 
end of the larva is a flat plate bounded at its periphery by the 
circular band. This plate stretched across, like the head of a 
drum, is pierced near its middle by the anal opening of the 
digestive tract. 
The anterior ciliated band, or briefly, anterior band, takes a 
sinuous course over the sides of the larva. Its direction may 
be seen by an examination of the figure. If we start with the 
posterior limb of the band, posterior to the mouth, we see that 
it runs at the side almost parallel to the circular band. About 
