Poikilothermic animals overcome the stress of the changing environmental conditions to a considerable extent by making alterations in their metabolic activities. It is now well established that animals acclimated to low temperatures show a higher metabolic rate than animals acclimated to high temperatures. This has been reviewed extensively (Bullock, 1955; Precht, Christopherson & Hensel, 1955; Prosser, 1967; Rao, 1967; Vernberg & Vernberg, 1972). It has been shown to be true in the tropical earthworms, Megascolex mauritii (Saroja, 1961). Likewise in several other tropical forms like some molluscs, crustacea, insects and arachnids, the rate of oxygen consumption increased on cold acclimation, and the percentage increase in the rate was 15 % when measured at 20 °C and 30 % when measured at 34 °C (Carlisle & Cloudsley-Thompson, 1968). The compensatory changes in metabolic rate are also reflected in tissue respiration. However, such studies in various animals have shown that the degree of adaptability of different tissues to acclimating temperatures is variable (Bowler, 1963; Prosser, 1965, Vernberg & Vernberg, 1966). But in the earthworm it has been shown that tissue respiration shows compensatory change on acclimation to low temperature (Saroja, 1962). The changes in various metabolic activities occurring on cold acclimation in poikilothermic animals are considered to be under the control of the neural and neuroendocrine secretions (Rao & Saroja, 1963; Rao, 1966). In earthworms Lampito mauritii the neurosecretory cells of the cold-acclimated worms were deeply stained and more highly granulated than in normal worms (Rao & Saroja, 1963). Increase in the neurosecretory activity on cold acclimation initiates the liberation of neurohumours into body fluids which release active principles that control the metabolic activities in these animals (Rao, 1962,1963,1965, 1967). The effects of the body fluids of the cold-acclimated animals on the respiratory rate of the tissues of normal worms in vitro in Warburg flasks showed an increase in the respiratory rate of the tissues of normal animals (Rao & Saroja, 1963; Saroja & Rao, 1965; Vijayalakshmi, 1964).
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