The lymphoid tissues in the adult immune system serve as primary and secondary depots of cells being involved in the immune response. Which of the following lymphoid tissues is unencapsulated?

The respiratory, alimentary and genitourinary tracts are guarded immunologically by subepithelial accumulations of lymphoid tissues which are not constrained by a connective tissue capsule. These may occur as diffuse collections of lymphocytes, plasma cells and phagocytes throughout the body and the lamina propria of the intestinal wall, or as more clearly organised tissue with well-formed follicles as found in the Peyer's patches.