Digestion occurs over the whole body surface of the animal, which compensates for its lack of a gut by living in the digestive tract of vertebrates where food is abundant. Furthermore, the ectoderm of tapeworms is covered with tiny microtrichia , dramatically increasing the surface area over which digestion can occur. Most cestodes are very elongate and flattened, with some attaining staggering proportions. There is generally a clearly defined head region bearing a complex organ of adhesion known as the scolex . The scolex has a variety of suckers and hooks that keep the parasite locked on to its host.
Many tapeworms have a trunk divided into a succession of proglottids.
The proglottid resembles a true, annelid segment, but is in fact merely a ring of weakness in the body wall. Each proglottid represents a reproductive structure containing both male and female gonads, which can be broken off from the body to regenerate asexually into a new tapeworm, or produce eggs sexually. The majority of cestodes are two-host parasites. Like the trematodes, these are incredibly numerous and successful universal parasites found in the alimentary canal of every known vertebrate species, generally existing unnoticed. They are rarely a serious threat to health unless a major infestation occurs.