What is the adaptation to the environment in parasitic worms?

Parasitic worms are living organisms that live in another organism, called the “host”, and feed on its useful substances. That is, the worms use the host both as a habitat and as a source of food. Accordingly, worms have features of adaptation to this lifestyle. Firstly, their outer shell is very strong, it is not subject to gastric juice. Secondly, they have a simplified digestive system, since most of the nutrients are supplied to them in an accessible form.