The botryoidal and vaso-fibrous tissues of the leech Hirudo medicinalis have been studied morphologically and histochemically. It is concluded that the cells forming these tissues represent cells of the vessel and sinus walls which have swollen as a consequence of the accumulation of pigment granules in their cytoplasm. This swelling is much more pronounced in the cells of the botryoidal tissue. These tissue systems are in communication with each other and with the contractile and non-contractile haemolymph vessels.

Apart from the pigment granules, few cytoplasmic inclusions were seen in the cells of the botryoidal tissue, but numerous granules are present in the vaso-fibrous cells: these granules are about 1 µ in diameter and contain phospholipid. The brown pigment of the botryoidal tissue seems to consist of a masked iron compound, which probably occurs in combination with the protein which forms the framework of the pigment granule; the pigment of the vaso-fibrous tissue is green, and contains no iron, but reacts to the tests for bile pigments.

These tissues are considered to represent ‘kidneys of accumulation’ for the iron and the bile pigment which probably represent the by-products of a breakdown of the blood used as food. The amount of botryoidal tissue is significantly greater in old than in young leeches.

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