The caecum of articulate Brachiopoda

Abstract

Caeca, the tubular outgrowths of the mantle of endopunctate articulate brachiopods, which penetrate the shell to connect with the periostracum by a brush, have been examined under the electron microscope and subjected to standard histochemical tests. The brush consists of many tubes with a protein membrane, which permeate a thin canopy of calcite separating the periostracum from the distal head of a caecum; the tubes and space beneath the canopy are filled with mucopolysaccharide. Elongate core cells surrounded by flattened peripheral cells make up the caecal head. The core cells are usually distended with spheroidal inclusions of muco- and glycoprotein, particulate glycogen and protein, and minor lipid bodies. They hang freely into a lumen occupying the stalk-like proximal part of a caecum , while their distal surfaces beneath the brush are microvillous. The peripheral cells are charged with protein inclusions. The lumen with in the caecal stalk is formed after the differentiation of a caecum on the outer lobe of the mantle, as an adjustment to a thickening exoskeleton. Proximally, it is bounded by outer epithelial cells but it extends medially to the basement membrane overlying the connective tissue of the mantle and is continuous laterally with intercellular spaces. The mucopolysaccharide in the brush and beneath the calcite canopy is derived from the core cells, and although no continuous seepage through the periostracum to the shell exterior can be demonstrated, it probably acts as a cement when the periostracum is broken and also inhibits penetration of the shell by boring organisms. The core cells as the principal features of caeca are considered to be storage centres for chemical components circulating with in the mantle.