What are the main features of organisms using the bodies of other organisms as a habitat?

Along with the ground-air, water and soil habitats, there is one more – another organism. The creatures that use this environment are called parasites. In the process of evolution, they adapted to the harsh living conditions and developed some structural features. Firstly, parasites have a simple external structure – they lack sensory organs and limbs. But their skin is strong, so the parasites are not harmed by strong acids and stomach enzymes. Secondly, almost all parasite systems are reduced. The reproductive system is well developed. Thirdly, parasites have special suckers that allow them to “linger” in the host’s body.