Ix ALIMENTARY CANAL 261 
In the Siluroid, Amiurus, the striated fibres of the outer circular 
layer of the oesophagus are continued, although but sparsely, into 
the inner circular layer of the stomach. 
The submucosa (Fig. 157) lies between the muscular layer 
externally and the epithelial lining internally, and is charac- 
teristically developed in the stomach, and even more so in the 
intestine. Histologically, it consists of a framework of connective 
tissue, enclosing in its meshes masses of leucocytes (lymphoid 
tissue), some of which are amoeboid and migratory, and may 
even be found between the cells of the intestinal epithelium 
(including in some instances the cloacal epithelium), probably 
actively participating in the transmission of food material 
from the alimentary canal to the lymphatics and blood-vessels ; 
while other and somewhat similar, but larger, leucocytes (phago- 
cytes), are concerned with the elimination of waste substances or 
noxious micro-organisms. In addition to the diffused lymphoid 
tissue of the submucosa, special rounded or oval, and sometimes 
encapsuled, masses of this tissue (lymph follicles) are common in 
the intestinal wall (Fig. 157) of Acipenser, the Dipnoi and some 
Elasmobranchs, and are perhaps the only representatives in Fishes 
of the solitary follicles or “Peyer’s patches” of the higher 
Vertebrates. A mass of lymphoid tissue exists in the axis of 
the spiral valve of Acipenser, which has been compared with a 
similarly situated structure in Zepidosiren.' In some Elas- 
mobranchs a large lymphoid organ is imbedded in the submucosa 
of the oesophageal wall, while a local thickening of the tissue is 
met with in the pyloric sphincter. Protopterus is remarkable 
among Vertebrates for the extraordinary development of lymphoid 
tissue,” which, apart from its distribution in the submucosa, is 
abundantly present between the longitudinal and circular muscle 
layers, and the peritoneal and muscular coats of the intestine. 
In addition to the lymphoid tissue the submucosa contains non- 
striated muscle cells and plexuses of capillary blood-vessels, which 
in certain Loaches (e.g. Misgurnus), where intestinal respiration 
occurs, extend between the cells of the intestinal epithelium. A 
network of lymphatic spaces or vessels surrounds the blood-vessels. 
In some Elasmobranchs the small arteries of the submucosa of 
the stomach are provided with singular sphincter muscles, which 
1 Hyrtl, Lepidosiren paradoxa. Abhand. d. béhm. Gesell. d. Wiss. 1845, p. 629. 
2 Newton Parker, op. cit. 
