No.1.] THE DEVELOPMENT OF BALANOGLOSSUS. 13 
so that it now occupies a relatively different position in the 
interior of the larva. Instead of elongating to compensate 
for its protrusion posteriorly it has in fact become somewhat 
shorter. Due to these two changes, but principally to the 
former, we find, as compared with the preceding stage, a 
change of level of the alimentary canal. For instance, the 
highest point reached by the oesophagus (where it makes its 
bend) is considerably farther forward in Fig. 3 than in Fig. 4. 
The plane in which the oesophagus joins the stomach is 
in Fig. 3 far in advance of the level of the low limb of the 
anterior band, while in Fig. 4 they are in the same plane. 
Again, the junction of the stomach and intestine is in Fig. 3 
anterior to the circular band, but in Fig. 4 far behind the 
same band. 
If we turn now to some of the minor or less prominent 
changes, we find the most obvious of these to have taken 
place in the anterior ciliated band. The long, delicate tentacles 
of the earlier stages have entirely disappeared, leaving only 
traces behind them. The course of the anterior band, how- 
ever, is still to be made out, especially in hardened specimens, 
and is seen as a band irregular in outline, following the same 
general course as in the preceding stage. An examination 
of stages between those of Fig. 3 and Fig. 4 shows that the 
tentacles shorten, become thicker, and are gradually absorbed 
at their bases. The details of the change will be described 
later. 
The constriction in the middle of the larva has involved 
on the ventral side the ventral region of the circum-oral area, 
laterally the continuation of the same area, and dorsally the 
constriction necessarily involves the extra-oral area at the 
same level. 
Another change will be noted involving an important region 
of growth, vzz., the extra-oral region lying posterior to the 
horizontal limb of the anterior band and between this and 
the posterior band. As compared with the preceding stage, 
it will be seen that this region of the body has not only 
relatively but absolutely increased in length antero-posteriorly. 
This I regard as a point of much interest, because from this 
