Physical characteristics

The body of the tapeworms is usually dorso-ventrally flattened, narrow, and highly elongate. It resembles a tape, which may explain the etymology of both common and scientific names of the class ("cestus" in Latin means belt, girdle, or ribbon). The size range varies very much: from 0.02 in (0.6 mm) length of the cyclophyllidean Mathevolepis petrotschenkoi (parasite of shrews) to 98 ft (30 m) length of the pseudo-phyllidean Hexagonoporus physeteris (from the sperm whale). As a rule, tapeworms are whitish because as internal parasites living in darkness they do not possess any pigments.

Typically, the body of tapeworms consists of three distinct regions: scolex (plural scoleces), neck, and strobila (plural stro-bila).

The scolex (sometimes referred to as "head") is the anterior end of the body. Its major function is the attachment of the parasite to the wall of the intestine. By this reason, it may bear spines, hooks, glands releasing adhesive secretions, grooves, suckers, tentacles, etc., or various combinations of these depending on the ordinal or family affiliation of the worm. The suckers are the most widespread attachment organs (e.g., in Cyclophyllidea, Proteocephalidea, Lecani-cephalidea, and in some Tetraphyllidea). They are usually cup shaped, with powerful muscular walls, four in number, two dorsal and two ventral. The bothridium (plural bothridia) is an ear-shaped muscular outgrowth projecting sharply from the scolex and often possessing leaflike mobile margins. The bothridia are usually four in number; they might be sessile or situated at the end of elongate stems connecting them with the scolex. Bothridia occur in several orders (Trypanorhyn-

Gravid Taenia, a beef and pork tapeworm. Shows branched uterus full of eggs. Tapeworms occur in the digestive tract of every vertebrate species. They have no head, mouth, or digestive system, but absorb food from the host's gut through their body surface. (Photo by ©Biophoto Associates/Photo Researchers, Inc. Reproduced by permission.)

Gravid Taenia, a beef and pork tapeworm. Shows branched uterus full of eggs. Tapeworms occur in the digestive tract of every vertebrate species. They have no head, mouth, or digestive system, but absorb food from the host's gut through their body surface. (Photo by ©Biophoto Associates/Photo Researchers, Inc. Reproduced by permission.)

cha, Diphyllidea, Tetraphyllidea, and Tetrabothriidea). The bothria (singular bothrium) are simple longitudinal grooves on the scolex, two in number (e.g., in Pseudophyllidea).

The scoleces of the worms of many orders are characterized by the presence of an apical organ consisting mostly of muscular and (or) glandular tissue. In some tetraphyllideans, this organ is represented by a well-developed gland at the apex of the scolex. In the majority of the proteocephalideans, the apical organ has more or less a structure identical to that of the suckers (often referred to as "apical sucker"); however, there are species of this order with an apical organ transformed into a sac filled up of glandular tissue. An immense variability of the structure of the apical organ can be seen in the Cyclophyllidea, where it is usually marked as a rostellum. It is protrusible, often dome-shaped, and in the most common case provided by one or two rows of hooks. In some families, the rostellum can be withdrawn in a special muscular pouch (rostellar sac). The protruded rostellum penetrates deeply into the intestinal wall of the host, anchoring there by the crown of hooks situated on its top. In addition, some cy-clophyllideans may have accessory circles of spines or strongly

Light micrograph image of a fox tapeworm (Echinococcus multilocularis). The most common host is the fox, with intermediary hosts of mice and humans. (Photo by ©Nicole Ottawa/Eye of Science/Photo Researchers, Inc. Reproduced by permission.)

developed glands associated with the rostellum, also facilitating the reliable attachment to the intestinal wall.

The neck is the region of the body just posterior to the scolex. It is usually short. This is a zone of proliferation, containing numerous stem cells. The latter are responsible for giving rise of the strobila.

The strobila is posterior to the neck. It consists of proglottides arranged in a linear series. The proglottis is a distinct portion of the body containing a set of reproductive organs. The stobila may contain from few (two in Mathevolepis petrotschenkoi), several dozens (in the majority of tapeworms), or numerous (more than thousand in Taenia saginata) proglottides. Each proglottis starts its development at the neck, as a result of the division of the stem cells. Typically, the formation of proglottids one by one at the neck is a permanent process lasting the whole life of the tapeworm in the final host. Just posterior to the neck, the proglottides are short and narrow, containing undifferentiated cells (juvenile proglottides). With the appearance of a new proglottis at the neck, already formed proglottides are pushed in posterior direction, which coincides with their growth and the gradual development of the reproductive organs in them. After the juvenile proglottides, each strobila typically contains the following types of proglottides (from anterior to posterior direction):

• Premature proglottides, with primordia of genital organs only.

• Mature proglottides, with developed and functioning male and female genital systems (almost all tapeworms are hermaphroditic).

• Postmature proglottides, in which the uteri are filled of developing eggs and gonads gradually degenerate.

• Gravid proglottides, containing uteri with ripe eggs.

As a rule, the gravid proglottides having terminal position in the strobila detach after completing their development. They pass to the environment with the host's feces or disintegrate along their route and only eggs are released.

Formation of proglottides occurs in 11 of the orders listed above. However, the members of the orders Gyrocotylidea, Amphilinidea, and Caryophyllidea have only a single set of genital organs per body. Consequently, they have no proglottides. The Spathebothriidea exhibits an intermediate pattern of body organization: an internal multiplication of reproductive organs down the strobila occurs but no distinct proglottides are formed.

In the past, an interpretation of the body organization of the tapeworms was proposed, considering them as colonial organisms, i.e., each worm was believed to represent a linear colony of numerous zoids (individuals). In other words, each proglottis was recognized as an individual. This view is known as the "polyzoic" concept or theory. The majority of recent workers do not support it. However, the terms arising from this concept, "monozoic" (for cestodes with a single set of genital organs per body) and "polyzoic" (for cestodes with proglottides), are widely used for describing the organization of the body of the tapeworms. Thus, Gyrocotylidea, Am-philinidea, and Caryophyllidea are monozoic, and the remaining orders (excluding Spathebothriidea exhibiting intermediate features) are polyzoic.

Tapeworms lack a gut during all the stages of their development. They feed through the tegument covering the body. Under the tegument, there are several layers of superficial musculature (usually three). Inside, most of the body is made up of parenchyma. Powerful longitudinal muscular bundles, responsible for the movements of the body, pass along the entire strobila. They separate the parenchyma in the center of each proglottis from that in the periphery (the relevant parts of the parenchyma are named medullar and cortical). The nervous system is represented by paired ganglia situated in the scolex and arising from them major anterior and posterior longitudinal nerves, the latter running through the strobila. There are also numerous transverse commissures connecting longitudinal nerves and smaller nerves emanating from them and reaching to the musculature and the receptors.

The excretory system includes flame cells scattered in the parenchyma. Small ducts connect these cells with the major longitudinal canals of the system passing along the strobila. Usually, the longitudinal canals are two dorso-lateral and two ventro-lateral. In addition to the excretion of metabolic prod ucts, this system also eliminates the excess water from the body of the worm. By this reason, it also is known as osmoregulatory system.

As a rule, each mature proglottis (or each body of a mono-zoic cestode) contains one male reproductive system and one female reproductive system.

The male reproductive system includes testes, from one in the genus Aploparaksis (Hymenolepididae) to several hundreds in the genus Taenia (Taeniidae). Each testis is provided with a narrow outgoing duct (vas efferens). These ducts unite into a common wider duct (vas deferens), which transports the sperm to the male copulatory organ. The vas deferens leads into a muscular pouch (cirrus pouch) containing the copula-tory organ (cirrus). Along its course, vas deferens may form seminal vesicles before entering the cirrus pouch (external seminal vesicle) and (or) within it (internal seminal vesicle), or to be highly convoluted, in order to have greater sperm storage capacity. The cirrus is a muscular organ, often with spines on its surface. It is able to invaginate (to be withdrawn) in the cirrus pouch or to evaginate (project) through the pore of the cirrus sac.

The female reproductive system consists of ovary, vitel-larium, ootype, uterus, vagina, seminal receptacle, and ducts connecting them. The sperm enters the female reproductive system through the vagina during the copulation and is stored in the seminal receptacle. The ovary is variable in location, shape, and size. As oocytes mature, they pass from the ovary into the oviduct. A narrow duct coming from the seminal receptacle joins to the oviduct, which is the place of the fertilization. Vitellarium may be a compact organ (in Cyclophyllidea, Tetrabothriidea, and Nippotaeniidea) or may consist of numerous vitelline follicles scattered in the parenchyma and possessing outgoing ducts uniting into a common vitelline duct (in the majority of orders). The vitelline duct also is connected with the oviduct, and one or more vitelline cells join to each zygote. Together they pass into the ootype. It is usually surrounded by glandular tissue (known as Mehlis's gland) producing a secretion, forming a thin envelope encompassing the zygote and associated vitelline cells. The young eggs pass from the ootype through the uterine duct into the uterus where they complete their development. In the majority of the tapeworms, the eggs leave the final host together with the proglottis in which they have developed. However, some cestodes have uterine pores and eggs can be released one by one.

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