84 ZOOLOGY 
spondingly large, in other forms the parenchyma occupies, 
with the exception of a few small lacunae full of fluid, the 
whole space within the integument; the digestive, nervous, 
excretory, and reproductive systems are embedded in it. 
The mouth opens on the ventral surface a little in front 
of the middle of the body. It leads into a pharynx whose 
wall, just within the mouth, is drawn out into two pharyngeal 
pouches. The next portion of the pharynx has a very thick 
muscular wall, and internal to this are two more pharyngeal 
pouches. The pharynx leads directly into the stomach, the 
demarcation between them being shown by the change in the 
lining epithelium, and by the presence of 
the salivary glands (Fig. 57). These 
consist of numerous pear-shaped gland- 
ular cells, with ducts which open at the 
junction of the pharynx and stomach. 
The stomach of Mesostoma is a simple 
sac, of uniform diameter. Owing to the 
position of the mouth near the middle of 
the body, the stomach may be divided 
into a short preoral and a longer postoral 
region. The stomach is lined by a single 
layer of cells, which vary much in shape, 
and appear to exhibit amoeboid move- 
ments similar to those of the endoderm 
in Hydra, and they ingest nutritious 
food-particles in the same intracellular 
way. No anus is present, and undigested 
matter is ejected through the mouth. 
The excretory system consists of (1.) 
Main trunks of Main trunks which open to the exterior, 
Fic. 58. 
the excretory system of (jj.) secondary branches given off from 
Mesostoma Ehrenbergii, 
0. Sch. After VonGraff. these, and (iii.) the excretory cells. 
They open on to the ex- 
In Mesostoma the main ducts open one 
terior close to the mouth. 
on each side close to the external pharyngeal 
pouches in the mouth (Fig. 58). From 
each opening a transverse duct runs dorsalward on each side of 
the pharynx and intestine; when it reaches the dorsal side of 
the alimentary canal it divides into two, one branch running 
1, Pharynx. 
