Several microanalytical techniques were used to assess the distribution and chemical form of chromium in various tissues of Mytjlus edulls, at the cellular and subcellular levels After 2 wk exposure to Cr (111) salt (Cr occurs principally in the trivalent state in the natural environment), investigations were performed on mussels, using secondary ion mass spectrometry (ion microscope and ion microprobe) associated with photon microscope, and X-ray spectrometry (electron microprobe) associated with transmission electron miscroscope. Cr measurements gave the following: kidney and gills exhibited the highest values, intermediate values were found for muscle and byssus (where Cr was adsorbed onto the threads and was incorporated within them), and lowest values were found in the digestive gland. Cr was also detected in the amoebocytes but not in the reproductive cells. The target organelle of Cr accumulation was shown to be the lysosome where the metal was associated with phosphorus and sulfur and trapped in an insoluble form. The significance of these different data is discussed. Cr kinetics of metabolism was compared between mussels and other aq.uatic organisms. Comparison in M. edulis of Cr metabolism with metabolism of other metals leads to the conclusion that Cr is metabolised and transported differently from most toxic metals.

Summary— Ecotoxicological investigations were performed on two sets of biological models. The first one concerns marine pollution and was composed of invertebrates (molluscs and crustaceans) contaminated by stable or radioactive elements originating from wastes discharged into sea water. The second one concerns freshwater pollution and was composed of vertebrates (fish) contaminated by aluminium which was dissolved in rivers, as a consequence of an atmospheric pollution by acid rain. Mechanisms involved in the uptake, storage and elimination processes of these toxicants were studied, with a special emphasis on cellular and subcellular aspects of concentration sites. Two microanalytical methods were employed: secondary ion mass spectrometry (SIMS), using the ion microscope and the ion microprobe, and X‐ray spectrometry using the electron microprobe (EMP). SIMS, which enables the visualization of trace elements, was associated with an image processing system using a highly sensitive television camera connected to an image computer. Polychromatic images were obtained, allowing to establish the cellular distribution of metal contaminants. In marine organisms, the target organs and tissues of Al, rare earth elements (Tm and La) and radionuclides (U, Pu, Am) were shown to be mainly digestive gland and exoskeleton. The target organelles were shown to be spherocrystals and lysosomes where the enzymatic lysosomal coprecipitation with phosphorus was observed. Amoebocytes, which are enzymatically equipped with lysosomal phosphatase, were involved in the phagocytic clearance of metal pollutants. In trout, two processes appeared to be involved in Al accumulation. The first one corresponds to the well known insolubilisation of Al phosphate, within lysosomes of organs devoted to uptake and excretion such as gill and kidney. The second one demonstrates that organs and tissues which cannot eliminate, such as bone, heart and brain, retain Al, exhibiting a high intracellular metal concentration; moreover, large Al deposits inducing nervous tissue destruction have been observed. Data have been discussed in connection with the relationship between man and his environment.

Experimental infestations of young Carcinus with Sacculina carcini indicate that the ventral ganglionic mass (VGM) is the first "target" of the parasite roots into the host tissues. As in pubescent crabs naturally parasitized, either with or without an external visceral sac, the roots penetrate and invade that area of the central nervous system during the first month of infestation.Ultrastructural study of the developing roots, in contact with the VGM, leads to the conclusion that apart from a few embryonic characters, the root cells show a cytostructure similar to the one that has been observed in the roots naturally parasitizing pubescent crabs, whether the external visceral sac is present or not.In the two cases of infestation the roots which cross the neurolemma bring about a marked disorganization of ganglia in consequence of the alteration of the neuroglia, neuropiles and neurosecretory areas.An action of the parasite at a distance, i.e. in the absence of contact of the roots with the ganglia, also occurs. It is especially marked as early as 1 month after infestation by a degeneration of the secretory perikarya.A thin sheath of connective tissue is sometimes visible between the growing roots and the VGM of juvenile crabs. However, the existence of that tissue cannot be definitely attributed to a host defense reaction.No effect of the developing parasite on the already differentiated external sex characters of the host has been noticed within the limits of the experiments.

A comparative examination of the free roots of Sacculina carcini, parasitizing Carcinus maenas, and Loxothylacus panopei, parasitizing Rhithropanopeus harrisii, shows numerous similar ultrastructural features as well as a few specific variations. Proceeding inwards from the outside, the following structures can be seen: (1) a thin and uncalcified exoskeleton with irregular processes fitted with vesicules forming a dense network and opening in the host's haemocoel; the whole may also display deep infoldings, these characters promote an important increase of the root surface; (2) subcuticular spaces sometimes particularly developed; (3) root cells that show a double aspect.In periphery, the electron-lucent cytoplasm with microtubules and free ribosomes, show apical villi which can penetrate the exoskeleton; mitochondria are abundant and possess a dense matrix with parallel cristae. Near the axis of the root, an electron-dense cytoplasm contains numerous rough-surfaced endoplasmic reticulum cisternae and large reserve inclusion bodies. Pictures of cytolysis are visible at the edge of cells. Middle lumen seem to appear after degeneration of some axial cells. Peculiarities of the described structures and their respective role in the absorption of substances, storage of reserves, or excretion of metabolites are discussed with regard to the host–parasite relationships.