The answer relates to the medium in which the sensed molecules travel. On land, animals detect a scent through organic molecules transported in the air. Land animals will experience a taste sensation by grasping an object using their mouth. Here, the organic molecules providing the sensation are transported by saliva or organic fluids of the object. So on land, there is a clear distinction between molecules transported in the wind and those that can be tasted. Water is a much more dense medium than air, and can carry small and large organic molecules, unlike air. Therefore, aquatic animals have little need for olfactory organs as all molecules can be 'tasted' from the water.
The immune system acts as a defense system of the body. It fights against bacteria, viruses and any other disease causing pathogens. Some chemicals can weaken the immune system and make them vulnerable.
