Aim: The present study was aimed to evaluate the effects of various drugs on different in habitat crabs Methods: The sterile syringes was loaded with different chemicals and poured over the heart of the dissected animal. The stop watch was switched on again and the number of beats per minute was noted. The rate of heart beats was plotted in relation to time exposure in four chemicals like Adrenaline, Atropine, Ephedrine and Ringer solution. The plotted graphical interpretation has been given the nature of chemical and their intensity and finally removed the hearts from the treated animal were subjected to histopathology and haemolymph preparations. Results: The hearts of the three different crustaceans were isolated for the histopathology process to study, diagnose and to differentiate between the cardiac muscles of the crabs. It was observed that edema was seen in all the smears but it was higher in marine species than the estuarine and freshwater species. Inflammation was also seen but except marine species, the others show inflammation. There were no toxic accumulation and pathogens observed and haemolymphs was observed that the smears were even and contains many cells which are irregular in their shape and size. Conclusion: From the results of study we conclude that, Adrenaline, Atropine, Ephedrine and Ringer solutions initially inhibit the heart rate which indicates animal try to settle-down into the chemical medium but later on the effects of toxicants accelerate the metabolic activity of the animals which accelerate the rate of heart beat in a crab after this treatment. Then the heart rate slowly decreases as the animal dies.