The ultrastructual morphology of the midgut diverticulum of the calanoid copepod Calanus helgolandicus (Claus) (Crustacea)

Abstract

The midgut diverticulum of the marine calanoid copepod C. helgolandicus consists of a columnar epithelial layer, a myoepithelial layer, and between these a well-developed basement membrane. The apical region of the epithelial cell is thrown into tightly packed microvilli which showed an alcian blue reaction indicating the presence of acid mucopolysaccharides. The apical half of the cell contains numerous microvesicles and mitochondria as well as tiny Golgi-like bodies. The plasma membrane of the basal region of the epithelium is extremely digitated. The digitations contain numerous mitochondria and the whole structure resembles mitochondrial pumps. The epithelial cells contain a large centrally situated oval nucleus with its single nucleolus. The myoepithelial cell is squamous and contains a flattened nucleus as well as very well-developed circularly and longitudinally arranged myofibrils. It is suggested that the midgut diverticulum of Calanus is probably analogous to the mammalian stomach in that food is mechanically churned. However, it does not appear to be involved in the secretion of digestive juices but only mucopolysaccharides; it is probably involved in the absorption of amino acids which are probably actively transported, by the "mitochondrial pump" in the basal region of the epithelial cells, into the haemocoel.