The parasitic flukes have a relatively thick integument, which bears many spines and sensory papillae, an apical membrane that is thrown into ridges and pits, and microvilli. The outer part of the integument contains secretory bodies, which are continuously released at the surface to renew the apical membrane. This appears to be a protective device for the parasite related to the immune reaction of the host. Roundworms have a thick, flexible cuticle, with three distinguishable zones, covered by an epicuticle.
Annelids
Annelids have a thin, horny cuticle pierced by pores through which epidermal glands secrete mucus. In some marine annelids, glands are also present that secrete materials constituting a parchmentlike or calcareous tube within which the worm dwells. Earthworms and leeches secrete cocoons from a specialized epidermis in a region of the body known as the clitellum. A major feature of all annelids except leeches is the possession of bristles, or chaetae, of which there are many varieties. The bulk of each chaeta is secreted by a single cell, though the surrounding lateral cells may contribute materials that bring about its hardening.