No.1.] THE DEVELOPMENT OF BALANOGLOSSUS. AI 
do not differ very much from the preceding stage. They are 
somewhat larger and the cells are in process of elongation. 
Fig. 39 is from a cross-section of a larva, cutting the surface of 
the third body-cavity, and shows the cells beginning to arrange 
themselves into fibres. This differentiation has gone farther 
in the last pair than in the collar-cavities. 
The next stage represented by Fig. 5, Pl. I, shows in serial 
sections that the changes inaugurated in the preceding stage 
have gone farther. Few new changes have begun, and there- 
fore, after a brief description, we may pass to a still later 
stage. 
The absorption of the tenacles continues. A cross-section 
of the proboscis of the larva is drawn in Fig. 40. The tentacles 
are seen in the figure on each side of the groove as two oblong 
masses fused over their whole length with the extra-oral ecto- 
derm. LEach is filled with many and exceedingly small nuclei. 
The anterior body-cavity has not yet filled out the cavity of the 
proboscis. Its walls are much thicker over the anterior two 
thirds, and the long ridges cut in cross-section appear projecting 
into the lumen. 
The first body-cavity is actually smaller than in the last 
stage, as may be seen in the surface views. 
Fig. 41 is drawn from a longitudinal section of a larva at 
this stage (perhaps a little older). The ectoderm has thickened, 
and is thicker over the body than over the proboscis. The 
digestive tract is seen in the center of the section. The mouth 
opens nearly at the general surface. The thick-walled oeso- 
phagus opens into the stomach and the stomach is separated 
from the intestine by a double fold of the wall of the digestive 
tract. The section passes to one side of the opening between 
the two cavities. 
The anterior body-cavity nearly fills up the interior of the 
proboscis, pushing before it the scattered mesenchyme tissue. 
Some of these are eventually caught between the mesodermal 
and ectodermal walls. Their subsequent fate I have not been 
able to trace with certainty. The section passes a little to one 
side of the proboscis vesicle. 
