INVERTEBRATE BLOOD-CELLS - MORPHOLOGICAL AND FUNCTIONAL-ASPECTS OF THE HEMOCYTES IN THE POND SNAIL LYMNAEA-STAGNALIS

The internal defence system (immune system) of the pond snail Lymnaea stagnalis is reviewed. Humoral defence activities are agglutination, o psonisation and inhibition of bacterial growth. Cellular defence is ex erted by antigen trapping endothelial cells, foreign protein engulfing pore cells, phagocytic reticulum cells and mobile haemocytes. The hae mocytes contribute most to defence and are therefore treated in more d etail. There is one type of haemocyte, morphological heterogeneity of the haemocyte population is due to varying states of differentiation o f the cells. Haemocytopoiesis is through mitosis of haemocytes, both i n the pool of tissue-dwelling cells and in circulation. Haemocytes res emble monocytes/macrophages, they are typical phagocytes equipped with recognition factors (lectins), lysosomal enzymes and a cytotoxicity m echanism using reative oxygen intermediates. A comparison is made of t he internal defence systems of the three main metazoan taxa, insects, molluscs and vertebrates.