STRUCTURAL ORGANIZATION AND EPITHELIAL-CELL TYPES OF THE INTESTINAL DIVERTICULA (PROTOPANCREAS) OF AMMOCOETES OF SOUTHERN-HEMISPHERE LAMPREYS - FUNCTIONAL AND PHYLOGENETIC IMPLICATIONS

Larvae of the two southern hemisphere lamprey genera, Mordacia and Geo tria, possess one and two intestinal diverticula, respectively, each o riginating at the oesophageal-intestinal junction. These diverticula c omprise an inner layer of simple columnar epithelium composed solely o f zymogen and mucous cells, a middle layer consisting mainly of a bloo d sinus, and an outer serosa layer covered by a simple squamous epithe lium (mesothelium). The inner surface is highly folded only in Mordaci a. The secretion of mucus probably protects the epithelium from the ef fects of digestive enzymes secreted by the zymogen cells and/or bile, which enters the diverticulum at its tip. Unlike the situation in sout hern hemisphere lampreys, the zymogen cells of the larvae of holarctic lampreys are located in the anterior intestine, a condition considere d to be ''primitive''. It is thus proposed that intestinal diverticula were developed during the evolution of southern hemisphere lampreys. The relocation of zymogen cells in the diverticula increases the area for these cells, and thus the capacity for the synthesis and secretion of digestive enzymes, particularly in Mordacia where the inner surfac e is folded.