Eucestoda, commonly referred to as tapeworms, is the larger of the two subclasses of flatworms in the class

Cestoda Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodies consist of ...

(the other subclass is Cestodaria). Larvae have six posterior hooks on the

scolex Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodies consist of m ...

(head), in contrast to the ten-hooked Cestodaria. All tapeworms are

endoparasites Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...

of vertebrates, living in the digestive tract or related ducts. Examples are the

pork tapeworm ''Taenia solium'', the pork tapeworm, belongs to the cyclophyllid cestode family Taeniidae. It is found throughout the world and is most common in countries where pork is eaten. It is a tapeworm that uses humans as its definitive host and pigs ...

(''Taenia solium'') with a human

definitive host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' ( symbiont). The guest is typically provided with nourishment and shelter. Examples include ...

, and pigs as the secondary host, and ''

Moniezia expansa ''Moniezia expansa'' is commonly known as sheep tapeworm or double-pored ruminant tapeworm. It is a large tapeworm inhabiting the small intestines of ruminants such as sheep, goats and cattle. It has been reported from Peru that pigs are also inf ...

'', the definitive hosts of which are

ruminant Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...

Body structure

Adult Eucestoda have a white-opaque dorso-ventrally flattened appearance, and are elongated, ranging in length from a few millimeters to 25 meters. Almost all members, except members of the orders

Caryophyllidea The Caryophyllideans are a group of tapeworms that infect fish and annelids (segmented worms) with a simple scolex or "head." Worms in this order only have one proglottid, which is believed to be the primitive condition for tapeworms. They are ge ...

and Spathebothriidea, are polyzoic with repeated sets of reproductive organs down the body length, and almost all members, except members of the order Dioecocestidae, are protandral hermaphrodites. Most except

caryophyllidea The Caryophyllideans are a group of tapeworms that infect fish and annelids (segmented worms) with a simple scolex or "head." Worms in this order only have one proglottid, which is believed to be the primitive condition for tapeworms. They are ge ...

ns consist of a few to 4000

proglottid Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodies consist of ...

s (segments) that show a characteristic body differentiation pattern into scolex (head), neck, and strobila.Mehlhorn, Heinz. "Eucestoda Classification." Encyclopedia of Parasitology. 2008. p. 495-497 The scolex, located at the anterior end, is a small (usually less than 1 mm) holdfast organ with specific systems for fastening itself to materials: rostrum, acetabula, suckers, bothria, grooves, and hooks. The small neck region, directly behind the scolex, consists of an undifferentiated tissue region of proglottid proliferation, leading into a zone of increasing and continuous proglottid differentiation. As such, the main and largest section of the body, the strobila, consists of a chain of increasingly mature proglottids. These cytological processes are not well understood at present. Members of the Eucestoda have no mouth or digestive tract, and instead absorb nutrients through a layer of

microtriches Microtriches (singular microtrix) are the highly specialized microvilli covering the entire surface of the tegument of cestodes. They are fine hair-like filaments distributed throughout the surface of the body, both unique to and ubiquitous among ...

over the tegument at the shared body wall surface.Rohde, Klaus. "Eucestoda." AccessScience. McGraw-Hill Ryerson In addition to the body wall, several other systems are common to the whole length of the tapeworm, including excretory canals, nerve fibers, and longitudinal muscles."Encyclopedia of Parasitology" The excretory system is responsible for osmoregulation and consists of blind-ending flame bulbs communicating through a duct system. The nervous system, often referred to as a "ladder system," is a system of longitudinal connectives and transverse ring commissures."AccessScience"

Reproduction

The reproductive systems develop progressively along the differentiated

proglottids Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodies consist of ...

of the strobila region, with each proglottid developing one or two sets of sexual organs that differentiate at different times in a species-specific pattern, usually male-first. Thus, moving in the posterior direction of the continuously maturing proglottid chain, there are proglottids with mature male reproductive organs, then proglottids with mature female reproductive organs, and then proglottids with fertilized eggs in the

uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...

, a condition commonly referred to as "gravid."

Proglottids

An atrium on the lateral margin of each proglottid contains the openings to both the male and female genital ducts. Follicular testes produce sperm, which are carried by a system of ducts to the cirrus, an eversible copulatory organ that usually has a hypodermic system of spines and a holdfast system of hooks. The main specialized female reproductive organs are an ovary that produces eggs and a

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