46 MORGAN. [Vor. IX. 
These are hollow, and their walls are formed by the longitudi- 
nal muscle-fibres. A careful examination will show that at 
this stage a layer of circular muscle-fibres is formed in the 
outer layer of the wall. Sections tangential to the wall of the 
proboscis body-cavity, show most clearly this layer. In Fig. 55, 
taken from such a section, the two layers of muscle-fibres are 
indicated by the two series of parallel lines running at right 
angles. Those passing up and down the plate are the outer 
or circular muscles. The layer of circular muscles is formed 
from cells in the wall of the body-cavity, and not from the mes- 
enchyme cells that still lie outside of the walls. It is interest- 
ing to note that the development of the circular layer of 
muscles takes place at the time when the young worm first 
goes into the sand. 
The structure of the proboscis vesicle, and its relation to the 
proboscis body-cavity, has not essentially changed. Fig. 56 is 
taken from a cross-section of the larva, through the middle of 
the proboscis vesicle (p.v.). The ventral wall of the proboscis 
vesicle is turned back to some extent upon itself, leaving a 
square cavity with lateral wings, lying between the wall of the 
proboscis vesicle and the body-cavity. The space is filled with 
a coagulated fluid, and occasionally a mesenchyme cell is found 
in it. This space opens behind, directly into the blastocoel- 
cavity. The lateral wings leading from the central space, are 
filled with loose rounded cells. I have not definitely made out 
the origin of these cells.’ They may come either fromthe 
mesenchyme cells (which seems more probable) or from the 
walls of the body-cavity ; but the cells of the latter are always 
on the inner side, making this view improbable. A portion of 
the exit-tube appears in the upper left-hand corner of the 
figure. 
A longitudinal section through the base of the proboscis is 
drawn in Fig. 58. The reference-letters sufficiently show the 
connection between this and the last section. The postero- 
ventral wall of the proboscis vesicle,shows differentiation into 
quite a thick layer of muscle cells. Numerous mesenchyme 
cells lie in the blastocoel spaces around the organs at this 
level. (4 indicates anterior direction.) 
