Peritoneal hydatids of Echinococcus granulosus equinus of 9 months standing in BALB/c mice occurred as free cysts or cysts within cyst masses. Both showed wide variation in size and in host tissue response, and all had a well-developed laminated layer separating the host tissue response from the germinal layer. In the smallest cyst-mass cysts the host tissue response was present as remnants of the initial cellular attack involving eosinophils. Slightly larger cyst-mass cysts possessed a primary macrophage invasion which phagocytosed the remnants of the initial attack and also, though to little effect, the laminated layer material. In the largest cyst-mass cysts a second macrophage invasion, of monocyte origin, had commenced and transformation stages of these cells to macrophages were observed. No fibroblasts surrounded individual cyst-mass cysts but they were present around the cyst mass, encapsulating it and possibly preventing further host cell invasion. Thus, the host tissue response around individual cyst-mass cysts remained 'preserved' at an early stage such as existed at the time of encapsulation. Small free cysts showed a primary macrophage invasion and transformation stages of cells of a secondary infiltration of peritoneal origin. Peripheral to the macrophages were fibroblasts demonstrating limited fibrinogenesis, and each cyst was surrounded by a layer of mesothelial cells. Large free cysts, also delimited by a mesothelial layer, possessed peripheral connective tissue, a deep fibrous layer and a monolayer of very compressed macrophages lying adjacent to the laminated layer. It is emphasized that an understanding of the host tissue response in cysts of different sizes and from different locations is an essential pre-requisite for the design of experimental studies.

The lead content of whole earthworms, highest in contaminated site specimens, was significantly higher in Dendrobaena rubida than Lubricus rubellus and a species difference in zinc was also recorded. Selective feeding and differential absorption are discounted and a species difference in maximum tolerance to body lead is suggested. Copper was low in all specimens. Chloragocytes and intestinal tissue showed significantly higher lead levels in contaminated earthworms than in control material. Ultrastructurally, chloragocytes from contaminated earthworms showed electron dense flecks associated with the chroragosome peripheries and within the debis vesicles. Very fine flecks occurred in the nuclei, but mitochondria and Golgi were indistingushable from control material. Preliminary X-ray microanalysis of contaminated chloragocytes revealed lead and phosphorus. The deposits within the chloragocytes might represent unbound lead precipitated by phosphate buffer; flecks being absent from contaminated, citrate buffered material and from control material. The chloragosomes are proposed as possible sites for sequestered lead. Chloragocyte and intestinal glycogen levels were significantly higher in control material where the chloragocyte cytoplasm was rich in alpha-glycogen rosettes, these being absent from lead contaminated cells. The glycogen-lead correlation suggests that the metabolism of contaminated chloragocytes is directed towards lead sequestration, though differing nutritional states cannot be ignored.

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SUMMARY The efficacy of albendazole (50 mg/kg/d), mebendazole (50 mg/kg/d) and praziquantel (500 mg/kg/d) against established intraperitoneal infections of Echinococcus multilocularis in gerbils was compared by monitoring parasite weight and making ultrastructural observations on treated and untreated material. Praziquantel was the most active protoscolicidal agent, reducing protoscolex viability to <2%, although it did not inhibit cyst growth. Albendazole was the most effective agent in reducing cyst growth and was, when compared with other regimes significantly more effective than mebendazole (p<005), praziquantel (p<001) or untreated controls (p<001).

The typically annelid cuticle of Phallodrilus albidus has an increased population density of epicuticular projections, but this is not thought to be related to its gutless condition. The irregular outline of the apical membrane of the epidermal cells is an inherent feature, possibly enhancing an absorptive function. The cuticle-epidermal interface is characterized by multigranular bodies, rod-shaped bacteria and filiform structures. The multigranular bodies are present in the postclitellar region only, coinciding with the chalkiness of living specimens. The bacteria occur throughout the entire length of the worm, their presence exaggerating the irregularities of the apical membrane. The bacterial cytoplasmic membrane, in contrast to the limiting membrane, was difficult to resolve. Between the two membranes is an amorphous zone. Storage granules of a carbohydrate-rich complex are located peripherally and the bacterial core contains a reticulum of DNA threads. Filiform structures, ultrastructurally similar to the bacterial peripheral region, are numerous, especially where the bacteria are carbohydrate-rich. Connections with the bacterial bodies were observed, but infrequently. The non-pathologic association between the subcuticular bacteria and the gutless tubificid is discussed.