ADAPTIVE FEATURES OF GUT STRUCTURE AND DIGESTIVE PHYSIOLOGY IN THE TERRESTRIAL ISOPOD PHILOSCIA MUSCORUM (SCOPOLI) 1763

¹ Department of Pure and Applied Zoology, University of Leeds, Leeds, England, U. K.

1. The alimentary canal in the terrestrial isopod Philoscia muscorum consists of a foregut (oesophagus and proventriculus), a midgut reduced to four simple caeca, and a hindgut subdivided into anterior, papillate and sphincter regions and a rectum. The anterior region has a dorsal typhlosole whose lateral extensions form two parallel channels beneath the roof of the hindgut.

2. The isopod feeds on decaying leaf-litter. Liquids are separated in the proventriculus and passed into the caeca and absorbed; solids receive a small amount of digestive secretions containing A- and C-esterases, from the caeca, as they pass into the anterior hindgut.

3. Microorganisms already present in the food and responsible for its degradation prior to ingestion continue their action in the hindgut. They produce cellulases demonstrable by a substrate-film technique and it is believed that they are the only source of the cellulases present in the alimentary system. There is some indication that microbial activity is enhanced after ingestion by the isopod.

4. Microbial attack on the food in the anterior hindgut lasts for 20-24 hours under laboratory conditions and is followed by the major discharge of caecal enzymes. These now bypass any food remaining in the anterior hindgut, or recently ingested and passed into it, by being carried along the dorsal typhlosole channels. They are discharged into the papillate region and attack food which has already been subjected to extended microbial action.

5. Absorption occurs in both the anterior and papillate regions of the hindgut. It is suggested that only products of cellulose digestion are absorbed in the former while other digestive products are absorbed in the papillate region.

6. The typhlosole and subdivision of the hindgut are interpreted, therefore, as adaptive modifications related to a diet of decomposing leaf-litter and the utilization of the digestive capabilities of normally free-living microorganisms for the breakdown of this food, and especially of its cellulose component.

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