The submucosa (Fig. 157) lies between the muscular layer externally and the epithelial lining internally, and is characteristically 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 (phagocytes), 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_.[238] In some Elasmobranchs 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,[239] 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 {262}occasionally encircle both the artery and the corresponding vein.[240]