Adaptive features of gut structure and digestive physiology in the terrestrial isopod philoscia muscorum (scopoli) 1763

TLDR: Little is known of the functional significance of the reduction of the midgut to four or six caeca and, in terrestrial species, of the subdivision of the hindgut and the development of a typhiosole, but it would seem likely that the latter modifications are related in some way to the terrestrial habit, probably as adaptations to the type of food available.

Abstract: The alimentary canal of isopod crustaceans is comparable to that of most other Arthropoda in that three basic regions can be recognized. These are the foregut, derived from the ectodermal stomodaeum and consisting of an oesophagus and pro ventriculus, the endodermal midgut, and an ectodermal hindgut derived from the proctodaeum (Goodrich, 1939) . The midgut, though, is unusual in that it is vir tually restricted to four or six simple caeca which arise at the junction of the fore- and hindgut. Controversy over whether any other part of the alimentary canal is of endodermal origin, and therefore to be regarded as midgut, has been resolved by Holdich ( 1973) who concludes from a comprehensive review of the literature that the discontinuity between the chitinous intimas of the fore- and hind gut described by Goodrich ( 1939), underlain in some species by a band of endo derm two to three cells wide, is the only remnant of a true midgut in isopods, other than the caeca. The terrestrial isopods show further modifications of the alimentary canal in that the hindgut is subdivided into an anterior somewhat expanded region, which bears along its length a dorsal typhiosole, a median papillate region, a muscular sphincter, and a short posterior rectum (Sutton, 1972). The histology and ultrastructure of the isopod gut is known from a consider able number of studies, summarized in Jones, Babbage and King (1969), Schmitz and Schultz (1969), Holdich and Ratcliffe (1970), Clifford and Witkus (1971) and Alikhan (1972). The caeca have received particular attention and most authors agree that they are the source of endogenous digestive enzymes which act on the food within the caecal lumen, the proventriculus, or the hindgut. The caeca are also concerned with absorption of digested food and in some instances are believed to carry on digestion intracellularly. Little is known, though, of the functional significance of the reduction of the midgut to four or six caeca and, in terrestrial species, of the subdivision of the hindgut and the development of a typhiosole. It would seem likely, however, that the latter modifications are related in some way to the terrestrial habit, probably as adaptations to the type of food available. The terrestrial isopods have a large proportion of cellulose in their diet, feeding on moist vegetable litter and often supplementing this by consuming their own, faeces (Wieser, 1966; Sutton, 1972). They show strong preferences for partly decomposed litter rich in the micro organisms which effect its decomposition (Edwards, 1974: Hassall, 1975a) and their gut modifications may well be adaptive features facilitating efficient utilization of this food, possibly by allowing the isopods to make extensive use of the cellulose degrading properties of the litter microflora. Endogenous cellulases are not corn