50 MORGAN. [Vou. IX. 
The cells of the anterior body-cavity as shown by dissections 
are drawn out into exceedingly long fibres, which are arranged 
in longitudinal rows. Each row runs the length of the pro- 
boscis and projects far into its interior. Outside of the longi- 
tudinal fibres a plate (or layer) of circular muscles is present, 
but one cell in depth. 
The collar-cavities are opening, but the lumen of this pair 
always remains filled with scattered, elongated cells. 
Each of the collar-cavities sends forward a median solid 
protrusion almost as far forward as the proboscis vesicle. 
The walls of the third body-cavity are formed of elongated 
cells, with the nuclei on the inner surface of the longitudinal 
fibre-like cells. Each body-cavity sends forward a protrusion 
in the middle line beneath the collar, as far forward as the 
beginning of the forward extension of the collar-cavities. 
We may pass to a stage when the young worm has reached 
the stage figured in Fig. 7, Pl. I. A drawing of a longitudinal 
dorso-ventral section of the worm is shown in Fig. 64, drawn 
to the same scale as the living larva of Fig. 7. The section 
passes through the median dorsal line, so that the nervous 
system is represented by that portion of ectoderm lying beneath 
and detached from the collar (above). The large mouth opens 
to the surface at the base of the proboscis, and leads into a 
spacious oesophagus. The stomach intestine fills up most of 
the interior of the body. The walls are thick anteriorly, but 
quite thin behind; and in the median posterior point the anus 
opens to the exterior. About the middle of the body the 
circular band is cut, and its position is marked by pigmented 
cells in the ectoderm. 
The body-cavities of the collar and trunk are not well shown 
in such a median section, but here and there portions are cut, 
as indicated by the darker lines in the figure. 
At the base of the proboscis, and in close proximity to the 
dorsal wall of the oesophagus, lies the proboscis vesicle. Its 
ventral wall is seen to be extremely thick, due to a prolifera- 
tion of cells into its interior. Touching the posterior wall 
of the proboscis vesicle is the median extension of the collar 
