Acanthocephala is a parasitic worm phylum that is distinguished by the presence of an eversible proboscis armed with spines that it uses to pierce and hold the gut wall of its host. Acanthocephalans have complicated life cycles that involve at least two hosts, which can be invertebrates, fish, amphibians, birds, or mammals. Acanthocephalans do not have a mouth or an alimentary canal. This is a trait they share with the cestoda, despite the fact that the two groups are not closely related. Adult stages live in their host's intestines and directly absorb nutrients digested by the host through their body surface. The presence of an anterior, protruding proboscis covered with spiny hooks is the most distinguishing feature of the acanthocephala. The proboscis has rings of recurved hooks arranged in horizontal rows that the animal uses to attach itself to the tissues of its host. The acanthocephala's body surface is unusual. The skin has a thin tegument that covers the epidermis, which is a syncytium with no cell walls. A series of branching tubules containing fluid traverse the syncytium, which is controlled by a few wandering amoeboid nuclei. There is no endothelium inside the syncytium, only an irregular layer of circular muscle fibres and some rather scattered longitudinal fibres.