ABSTRACT
The functional morphology of the digestive system in two amphibious and two terrestrial isopod species was studied by means of light and electron microscopy. In Ligia italica and Ligidium hypnorum the digestive canal comprises straight ectodermal foregut and hindgut and a short endodermal midgut with midgut glands. The structural features of the foregut in these species demonstrate that its functions mainly are to press, filter and channel the food to the midgut glands and the hindgut. The hindgut is a simple tube with loose muscular network, smooth cuticular lining and without typhlosole. Its main functions are transport and absorption of nutrients and formation of faecal pellets. The digestive canal of Trachelipm illyricus and Porcellio scaber consists of ectodermal foregut and hindgut and two pairs of midgut glands caeca. The foregut in terrestrial isopods comprising a well-developed masticatory apparatus, tooth-like cuticular spines and large atrium, is more complex than in amphibious species. It mainly triturates and filters the food. The hindgut, containing a prominent typhlosole, thick muscular layers, diversity of cuticular spines and abundant microflora, is divided into functionally different anterior and papillate regions. The hindgut is an important passage-way for nutrients, where food is both mechanically and chemically transformed and separated into fractions. Nutrients are partly returned to the foregut via typhlosole channels and mixed with digestive juices in the large atrium. The basal parts of hindgut cells of papillate region bulge into haemolymph lacunae. Ultrastructural features of the cells suggest that they are involved in ion and water transport. The endodermal part of the digestive system is restricted to two pairs of midgut gland diverticula in higher oniscideans. In primitive amphibious species an endodermal midgut and paired midgut gland diverticula are present. Ultrastructural analysis of the midgut and midgut glands demonstrates that they are involved in the secretion of digestive enzymes, absorption and storage of lipids respectively.
