IX ALIMENTARY CANAL 26 1 



In the Siluroid, Aviiurtis, 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 Lepidosiren} 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. Frotopteriis 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 



' Hyrtl, Lepidosiren paradoxa. Ahhand. d. lohm. Gesell. d. Wiss. 1845, p. 629. 

 ^ Newton Parker, op. cit. 



