Eucestoda

Eucestoda is the larger of the two subclasses of flatworms within the class Cestoda, the other being Cestodaria. Its members are called tapeworms. Unlike the ten-hooked Cestodaria, Eucestoda larvae (oncospheres) have six hooks on the hexacanth embryo, not on the scolex; the scolex develops later in the metacestode stage. All tapeworms are endoparasites of vertebrates, residing in the digestive tract or nearby ducts. Examples include the pork tapeworm (*Taenia solium*), which uses humans as its definitive host and pigs as secondary hosts, and *Moniezia expansa*, whose definitive hosts are ruminants.

Adult Eucestoda are white-opaque, dorso-ventrally flattened, and elongated, ranging from a few millimeters (about ¼ inch) to 15–20 meters (49–66 feet) in length. Almost all members—except those in the orders Caryophyllidea and Spathebothriidea—are polyzoic, meaning they have repeated sets of reproductive organs along the body. Nearly all, except members of the order Dioecocestidae, are protandral hermaphrodites. Most, aside from caryophyllideans, consist of a few to 4,000 proglottids (segments) and show a characteristic body plan: scolex (head), neck, and strobila. The scolex, at the front end, is a small holdfast organ (usually under 1 mm) with structures like a rostrum, acetabula, suckers, bothria, grooves, and hooks for attachment. Just behind it, the neck is an undifferentiated region where new proglottids form, leading into a zone of increasing differentiation. The main body, the strobila, is a chain of progressively maturing proglottids. These cellular processes are not well understood.

Eucestoda lack a mouth or digestive tract; they absorb nutrients through a layer of microtriches on the tegument, the shared body wall surface. Other systems run the length of the worm, including excretory canals, nerve fibers, and longitudinal muscles. The excretory system handles osmoregulation via blind-ending flame bulbs connected by ducts. The nervous system is a "ladder system" of longitudinal connectives and transverse ring commissures.

Reproductive systems develop progressively along the strobila’s proglottids. Each proglottid develops one or two sets of sexual organs that mature at different times in a species-specific pattern, typically male-first.