Tapeworms absorb dinners through skin while lacking a mouth
Cestoda packs some 6,000 parasitic flatworm species; the familiar ribbon adults are eucestode tapeworms built from repeating proglottids. Adults usually lodge in vertebrate guts for years, gripping with a scolex of hooks or suckers, while larvae cycle through insects, crustaceans, or other vertebrates.
Without mouths they absorb nutrients across the body surface. Humans meet Taenia from undercooked pork or beef, Diphyllobothrium from fish, and hygiene-linked Hymenolepis or Echinococcus. Complex life cycles demand at least one intermediate host before the definitive host stages the adult.
The architecture is ruthlessly modular: scolex, neck, then a strobilus that sheds egg-laden segments.
Every tapeworm is a reminder that evolution can discard guts and still feast—if hosts keep serving.
Size tracks the host. The beef tapeworm Taenia saginata can reach 20 metres and the whale tapeworm Tetragonoporus calyptocephalus exceeds 30, while parasites of voles and lemmings stay small. Lacking gut and mouth, cestodes take in food through a specialised outer layer, the tegument, which also handles gas exchange and shields them from the host's digestive enzymes. They cannot make their own lipids, so they rely wholly on the host for the fats needed to reproduce. The scolex is often tiny beside the rest of the worm, usually a four-sided knob armed with suckers, hooks or both, and a cerebral ganglion inside it serves as the main nerve centre. New segments bud from the neck and grow as they are pushed backward; mature ones are little more than sacs of eggs that leave in the host's faeces. Every cestode is a hermaphrodite. Diphyllobothrium runs a three-stage cycle whose larvae hatch in water and first infect copepods.
Source: Cestoda
