No.1.] THE DEVELOPMENT OF BALANOGLOSSUS. 27 
section through the middle of the body; occasionally, one is 
applied to the inner surface of the body-wall, or to the outer 
surface of the digestive tract. 
A cross-section of the oesophagus is shown in Fig. 5. It is 
elliptical in outline, and the internal walls (except at the sides) 
are richly ciliated. These ciliated cells are continuous with 
the ciliated bands above or below the mouth opening ; and, 
where the oesophagus opens into the stomach, the ciliated 
cells are continuous with the ciliated band on the anterior 
wall of the stomach. The outer wall of the oesophagus ts in 
part covered by a layer of muscle-cells (mesenchyme). These 
are found around the oesophagus at the point where the 
oesophagus joins the stomach, as shown in Fig. 5. The 
muscle-cells are elongated fibres, with the long axis around 
the oesophagus. Each fibre shows a strongly-refracting outer 
layer, and in the center lies the nucleus. These cells are the 
earliest muscle-cells and only ones to differentiate from the 
mesenchyme. The stomach is a large, thin-walled tube. A 
portion of its ventral wall, where it opens into the oesophagus, 
is shown in Fig. 4. The stomach is separated from the intes- 
tine by a constriction, leaving a central hole between the two 
cavities, richly ciliated. The walls of the intestinal division 
are thinner, and thrown into irregular folds. Posteriorly near 
(or in) the center of the posterior plate, the intestine opens by 
a small anus. 
The anterior, first, or proboscis body-cavity is present as a 
simple tube; it touches the surface of the digestive tract at 
the anterior end of the stomach, as seen in Fig. 3 at 6.c.1. The 
polygonal outlines of the cells of the stomach-wall are seen, 
and in the lower portion of the section the wall of the pro- 
boscis body-cavity is seen in surface view. The wall is formed 
by flattened cells, whose outlines are not clearly made out, 
the nuclei alone indicating the number of cells present. The 
body-cavity opens to the exterior near the mid-dorsal line, to 
the left, by a short (ectodermal) tube. The body-cavity sends 
forward a solid prolongation of its walls to join the under 
surface of the apical plate. Where the oesophagus joins the 
stomach, a pair of diverticula, hollow at first, are given off 
