MORPHOLOGY OF COMPLEX LYMPHOEPITHELIAL ORGANS OF THE ANAL-CANAL (ANALTONSIL) IN THE BOTTLE-NOSED-DOLPHIN, TURSIOPS-TRUNCATUS

A complex of lymphoepithelial organs, the ''anal tonsils,'' is a consi stent structure in the anal canal of the bottlenose dolphin, Tursiops truncatus. This complex occurs as a circumferential cluster of discret e tonsil-like aggregations of lymphoid tissues, together with epitheli al ducts (''crypts'') and occasional mucus secretory units in the extr eme lower portion of the intestinal tract. These structures are concen trated in the segment lined by stratified squamous epithelium and exte nd for a variable distance cephalad from the anal aperture. The tonsil s appear to be most active, judged by the amount of lymphoid tissue pr esent, in young animals. Depletion of lymphocytes and cystic enlargeme nt of the crypts, probably representing functional as well as morpholo gical involution, is a consistent feature of older animals. (C) 1995 W iley-Liss, Inc.