This group of parasites are referred to as "tapeworms". All species are endoparasitic. Almost all of the tapeworms belong to this taxon.
There are at least two, and occasionally three, hosts in the life cycle. Adult tapeworms live in the lumen of the digestive tract of a vertebrate definitive host. Larval tapeworms parasitize invertebrate or vertebrate intermediate hosts. There are three general types of life cycles. All transfer from host-to-host is via the food chain. There are no tapeworms that seek out and penetrate the body of any of their hosts.
In the first, the sequence of development isegg and coracidium (in water), procercoid (in invertebrate first intermediate host), plerocercoid (in fish second intermediate host), and adult (in fish-eating vertebrate definitive host). This pattern is exhibited by many of the major tapeworm taxa. Humans are involved in these life cycles as a definitive host.
In the second sequence, eggs are deposited on land or in water and are eaten by an invertebrate intermediate host, where they develop into a cysticercoid. The cysticercoid will infect a vertebrate definitive host. This pattern is exhibited but many species but they occur in only a few of the major taxa. Humans are usually involved in these life cycles as a definitive host.
The third sequence involves eggs deposited on land. These are eaten by a vertebrate intermediate host, where they develop into one of a variety of larval forms (cysticercus, coenurus, strobilocercus, hydatid) which are infective to a vertebrate definitive host. This pattern is also exhibited in many species, but from only a few of the major taxa. Humans may be involved in these life cycles as and intermediate host and a definitive host.
The most obvious unifying feature of the cestodes is that they lack a gut and have a solid body filled with parenchyma. There is no gut, because all digestion and absorption of nutrients occurs across the tegument which covers the body surface. The anterior end of the body is modified into a holdfast organ called a scolex, which may have a rostellum, suckers, bothria, bothridia, tentacles, hooks and/or spines to aid attachment to the gut of the host. Behind the scolex is a short neck region, which has germinative cells that give rise to the rest of the tapeworm body, or strobila. The youngest portion of the strobila is just behind the neck, while the oldest portion is at the posterior end of the body.
Almost all tapeworms are hermaphroditic. The male reproductive system comprises various numbers of testes, connected by vasa efferentia to a common vas deferens, which then enters a cirrus pouch and then a cirrus. The vas deferens may be enlarged for sperm storage outside and inside the cirrus pouch, called external and internal seminal vesicles, respectively. The cirrus, which delivers sperm to the female system, may be retractable and may be covered with spines.
The female reproductive system comprises a number of structures meeting in a region called the ootype. An ovary produces oocytes which are carried to the ootype via an oviduct. Vitelline glands are present as a field of separate follicles found in lateral fields, or as a single compact vitellarium located on the midline of the body. They produce nutritive and shell materials that are carried via vitelline ducts to the ootype where they are incorporated into eggs. Sperm are received via a vagina. A Mehli's gland may surround the ootype. Fully-formed eggs exit the ootype and accumulate in a uterus. The cirrus pouch and vagina usually emerge at a common genital pore, which may be on the ventral midline of the body or the side. The uterus often is a blind-ending sac, but when a uterine pore is present, it is usually separate from the genital pore.
The tapeworm body is usually externally segmented into a series of proglottids. Each proglottid contains one (and sometimes two) complete copies of the male and female reproductive organs. Tapeworms are definitely adapted for a high rate of reproduction! Sometimes, however, there is no external segmentation and the proglottids are separated internally only. Rarely, there is no segmentation and just a single set of reproductive structures. Because the youngest proglottids are near the neck, they have only rudiments of the reproductive organs and are referred to as immature proglottids. Older proglottids, in the middle of the strobila, have fully developed male and female reproductive organs, and are referred to as mature proglottids. In the oldest proglottids, at the posterior end of the strobila, eggs have been produced and fill the uterus, and the other reproductive organs begin to degenerate. These are called gravid proglottids. Because the development of proglottids is a gradual one from anterior to posterior, the distinction among immature, mature and gravid proglottids may be unclear in some parts of the strobila.
Because of the unusual body form of tapeworms, there are a number of descriptive terms that may be encountered when keying out a specimen. Proglottids that overlap the one behind them are called craspedote, while those that simply butt up against one another are acraspedote. Proglottids that detach from the body to disperse their eggs are called apolytic, whereas those that remain attached are called anapolytic. When there are multiple sets of reproductive organs, the genital pores may be uniform (all emerging on the same side of the body), alternate (the pore on each proglottid is on the opposite side of the ones in adjacent proglottids), or irregular (emerging in no particular pattern.)
The cestode oncosphere is undifferentiated except for the presence of three pairs of small hooks. The coracidium is simply an oncosphere surrounded by a layer of ciliated cells. The procercoid is ellipsoid in shape with penetration glands at one end and a cercomer at the other end. Plerocercoids are long and undifferentiated except for the scolex, which now has its adult form. Cysticercoids are solid-bodied larvae with a fully-developed scolex enclosed in a series of tissue layers, and an elongate cercomer. Bladder larvae have one or more scoleces developed within a fluid-filled capsule. The cysticercus contains a single, invaginated scolex. The coenurus has multiple invaginated scoleces. The strobilocercus has a single scolex attached to the bladder by a long section of strobila. Hydatids have a complex cyst containing internal (and sometimes external) brood capsules lined with a germinal epithelium, and numerous small protoscoleces.