On the ultrastructure of polypeptide hormone-producing cells in the gut of the ascidian, Ciona intestinalis L. and the bivalve, Mytilus edulis L.

Ultrastructural evidence has been found for the presence of polypeptide hormone-producing cells in the gut of Ciona intestinalis L. and Mytilus edulis L. which do not appear to have been described before. Due to their localization and ultrastructural characteristics, it is suggested that the cells in Mytilus edulis probably produce an insulin-like substance and that some of these cells in Ciona interstinalis may produce 5-HT (5-Hydroxytryptamine). In each species only one granulated cell type can be observed; The granules, which are electron dense and membrane bound, also show a halo. The average diameter of the granules is 100-200 nm for Ciona and 200-400 nm for Mytilus.