16 MORGAN. [VoL EX. 
region behind the circular band has increased in length and 
changed its general shape. It is now less conical than before 
and about as long as it is broad. The anus opens at the mid- 
posterior point of this region. The posterior band is conspicu- 
ous; it is marked by red-brown pigment and has extremely long 
cilia projecting from its surface. At times the cilia are actively 
moving and drive the body forward, at other times they le 
quietly at the sides. At this age the larvae when put into an 
aquarium sink immediately to the bottom, and, lying in a 
horizontal position, are driven along by the cilia. Wave-like 
contractions pass along the proboscis at times, although the 
proboscis is not used as yet as an organ of locomotion. 
The digestive tract is pulled still farther posteriorly so that 
the gill-pouches now lie at a level posterior to the constriction. 
The three divisions of the digestive tract are still found; and 
its walls are much thicker than in the earlier stages. 
The outlines of the anterior body-cavity cannot be seen in 
surface view, but from sections we see that it now fills com- 
pletely the interior of the anterior portion of the body, and its 
walls are in contact with the ectoderm. In the median dorsal 
line and slightly in front of the constriction, the anterior body- 
cavity opens to the exterior. The outline of the proboscis 
vesicle may be seen projecting into the body-cavity. 
The outlines of the posterior body-cavities were not seen 
in surface views in the living animals. In preserved larvae, 
cleared in oil of cloves, their outlines may be distinctly seen. 
These two pairs nearly encircle the body, but each one of a 
pair is separated from its fellow in the mid-dorsal and mid- 
ventral lines. The anterior pair— collar cavities —or second 
pair of body-cavities, are much smaller than the last or third 
pair. This last pair extends from in front of the circular band 
almost to the posterior end of the body. 
We come next to a stage where the larva has reached its 
minimum size. The individual drawn in Fig. 6, Pl. I, had 
been kept in an aquarium for twelve and a half hours, but it 
differs in no respect from other individuals of the same size 
that were caught in the tow-net at the surface of the Gulf 
