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 ceUs, 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 

 ia 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 (i.) 

 Fia. 58.— Main trunks of main trunks which open to the exterior, 

 'ms^:^\Z^i;l ("•) secondary branches given off from 

 0. Soh. After Von Graff, thesc, and (iii.) the excretory cells. 

 SlTe tTt^e llZ. I^ Mesostoma the main ducts open one 



1. Pharynx. ^^ ^aqh 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 



