With 7 plates and 4 figures in the text)The functional morphology of the digestive system of terrestrial isopods (woodlice) has been investigated. Observations have been made on live juveniles of Oniscus usellus and Porcellio scuber in which some digestive processes can be observed directly through the cuticle. The fine structure of the foregut of juveniles and adults of these two species has been studied by light microscopy and scanning and transmission electron microscopy. The passage of coloured dyes and colloidal gold through the digestive system has also been followed.The digestive system of Oniscus asellus and Porcellio scuber is divided into five regions. The foregut (l), and the anterior chamber (2), papillate region (3) and rectum (4) of the hindgut, are ectodermal in origin and lined with cuticle. The hepatopancreas (5), which opens into the foregut, consists of four blind-ending tubules which are endodermal in origin and not lined with cuticle. A powerful muscular sphincter is present between the papillate region and the rectum. There is no peritrophic membrane in the lumen of either the hepatopancreas or the hindgut.Fragments of food pass via the oesophagus to the proventriculus of the foregut where they are briefly masticated before passing rapidly into the anterior chamber of the hindgut. During this process, the food is mixed with secretions derived from the hepatopancreas. The anterior chamber possesses a pair of dorsal channels formed from a typhlosole and lateral infoldings of the gut wall. When the hindgut is full of food, contraction of muscles surrounding the gut forces liquids and fine food particles back into the foregut via the typhlosole channels which open at the junction of the anterior chamber and papillate region. This material, which contains fluids derived from the hepatopancreas, and products of digestion, is filtered in the foregut and passed into the lumen of the hepatopancreas where further digestion, and absorption of nutrients, takes place. The hepatopancreas may secrete these fluids back on to the food in the anterior chamber and receive products of digestion uiu the typhlosole channels several times during a digestive cycle. The lumen of the foregut, hindgut and hepatopancreas often contains a dense population of microorganisms which may be involved in digestion. Residual material in the hindgut passes eventually into the rectum where faecal pellets are compacted before they are voided.The ability of terrestrial isopods to recycle and retain fluids within the digestive system, by the development of the typhlosole channels (which are not present in marine isopods), may have been a major contributory factor in their successful transition from water to land.

The ultrastructure of the two cell types of the hepatopancreas of the terrestrial isopods Oniscus asellus and Porcellio scaber was examined at hourly intervals in animals habituated to a 16 h light: 8 h dark cycle. The ultrastructure of the B cells undergoes substantial changes which are repeated every 24 h. This 'B cell cycle' can be divided into two stages. During the first, 'extrusive' stage, which begins about 1 h before the onset of the light period, the contents of the B cells apical to the nuclei are voided into the lumen of the hepatopancreas. The second, 'restitutive,' stage begins about 10 h after the onset of the light cycle. The B cells swell in size by the accumulation of the new material until the next extrusive stage. The ultrastructural appearance of the S cells was similar at all stages of the daily and moult cycles; they were never observed to void any material into the lumen of the hepatopancreas. The differences between the rates of turnover of the contents of the S and B cells have important implications for understanding the dynamics of accumulation and loss of metal pollutants in terrestrial isopods.

: Porcellio scaber Latreille (Crustacea: Isopoda) of one month in age were reared for a year on leaf litter of field maple (Acer campestre) contaminated in the laboratory with a range of concentrations of cadmium, copper, lead or zinc. The metals were applied topically to the leaves as nitrates. Growth and survival, numbers of live offspring produced by females that matured, and concentrations of metals in adult isopods at the end of the experiment were measured.'Critical concentrations' of metals in food at which all the isopods died before producing offspring were 100 μg Cd g(-1), 100 μg Cu g(-1), 2000 μg Pb g(-1) and 1000 μg Zn g(-1) (on a dry weight basis). The relative toxicities of the four metals in the laboratory were compared with concentrations of cadmium, copper, lead and zinc in surface leaf litter in the vicinity of a primary smelting works at Avonmouth, South West England. The results support the hypothesis that the absence of Porcellio scaber from sites in the immediate vicinity of the factory is due to zinc poisoning. Although cadmium is approximately ten times more toxic to isopods than zinc in the laboratory, zinc is most likely to be killing isopods in the field because its concentration is always at least 30 times higher than cadmium in Avonmouth leaf litter, and more than 100 times higher at most sites.Populations of Porcellio scaber survive in field sites where surface leaf litter contains up to 5000 μg Zn g(-1). This is at least five times higher than the 'critical concentration' in laboratory experiments. Thus, the methodology for assessing metal toxicity described in this paper, exaggerates the potential effects of metals to isopods in the field. Such differences between laboratory and field toxicities of metals should be taken into account when environmental protection levels for metals are being proposed for soil invertebrates based on ecotoxicological tests conducted in the laboratory.

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