In those intestinal parasites, which do not absorb nutrients through the body surface, there is usually a thick cuticle. So helminths tend either to loose their gut and absorb nutrients through their teguments, or else retain their gut and have a thick resistant cuticle. In many parasites there is a tremendous elaboration of the reproductive organs, associated with increased gamete production. Cestodes, for example, basically consist of a small head and neck region and the rest is serially repeated gonads. Parasites can be described as being solely adapted for reproduction.
Parasitic protozoa are in an essentially isosmotic environment and so lack a contractile vacuole.
In parasitic insects there are often elaborate tracheal trunks, so the insect can remain air breathing even when it is in its host. There is usually an increase in reproductive potential compared with free-living relatives. Parasites usually produce more eggs and sperm than their free-living relatives do and there may be a great elaboration of the reproductive organs. Other adaptations, which increase egg production, are hermaphroditism and parthenogenesis, where every individual produces eggs and loss of seasonal reproductive cycles, so eggs and sperm are produced all the year round. Rapid maturation and extended life span also increase total reproductive capacity.