Histology and ultrastructure of the tube foot epithelium in the phanerozonian starfish, Astropecten

Abstract

The pointed tube feet of the primitive phanerozonian starfish Astropecten were examined by both light and electron microscopy. The cuticle overlying the whole tube foot is composed, in part, of acidic mucopolysaccharides, and ultra structurally consists of two layers: an inner layer of loosely packed fibers and a more dense fibrous outer layer. The outer epithelium of the tube foot tip possesses at least four distinct structural cell types: ciliated cells, vacuolated cells, cells containing small, dense spherical granules (80-160 mμ ), and cells with long rows of dense oval secretory granules (500 x 800 mμ The larger secretory granules are composed of both neutral and acidic mucopolysaccharides and lack the complex substructure of similar secretory granules in the suckered tube feet of more advanced starfish. The possible functions of the cuticle and cell types as well as the relationship between the ultrastructure, chemical nature, and function of the large secretory granules are discussed.