No.1.) ZHE DEVELOPMENT OF BALANOGLOSSUS. aE 
the exterior, while the extra-oral presents a convex surface 
to the observer. Coupled with this is the rolling over of the 
edge of the extra-oral area carrying the ciliated band, decreas- 
ing proportionately the distance between those bands that run 
parallel to one another. Owing to this rolling over, the bases 
of the tentacles lie below the general level of the extra-oral 
surface, as is shown to some extent in the figures. The course 
of the anterior band in the ventral region around the mouth is 
somewhat complicated, so that there is formed the arrangement 
that I have figured for this larva in my preceding paper 
(Fig. 12, Pl. XXV, JouRNAL oF MorpuHotocy). For a figure 
of the apical plate of the larva at this stage, see in the same 
paper(Pig sO nlo ke): 
In the apical plate the anterior ciliated band breaks into 
four free ends, not united across from right to left, and those 
on one side running parallel to one another. A surface view 
of the eyes of a larva of this stage is given in Fig. 19, 
Pl. III. (The figure of the apical plate given in my earlier 
paper does not properly show the eyes, owing to the fact that 
the pigment had been dissolved.) The structure of the eyes is 
treated in a later section of this paper. It will be noticed that 
the large pigment dots of the circum-oral area have entirely 
disappeared in the larger larva, although sections show that 
large, clear spots indicate their previous position. 
The posterior ciliated band is very conspicuous, and is the 
chief locomotor organ of the larva. Long and large cilia pro- 
ject from the ring, and waves of ciliary movement may be seen 
in the living larva to pass over the band. 
The posterior plate is still a flat ectodermal membrane 
stretching across the posterior end of the larva, and is pierced 
near its center by the anal opening. 
The digestive tract, which is small relatively to the size 
of the larva, is seen within the body. The oesophagus has 
lengthened, and is sharply bent on itself. No other changes 
of importance are noticeable in the digestive tract. 
The anterior body-cavity has increased enormously in size, 
so that it occupies a large portion of the interior of the 
anterior end. Its lumen extends forward almost to the apical 
