Illustrated Glossary



General features

The cestodes are a relatively diverse group of platyhelminths, with estimates of the total number of species exceeding 6,000. The class Cestoda is currently considered to include 18 orders, ranging from the enigmatic Cathetocephalidea, Spathebothriidea, and Nippotaeniidea, each with only 4 to 7 species, to the Cyclophyllidea, with well over 3,000 species. The cestodes are commonly referred to as the tapeworms owing to the ribbon-like form of the body of many species. The tapeworm orders other than the Amphilinidea and Gyrocotylidea comprise the subclass Eucestoda; despite the fact that they do not form a monophyletic group, the Amphilinidea and Gyrocotylidea are sometimes collectively referred to the subclass Cestodaria.

Tapeworms are entirely parasitic. Most occupy the digestive tract of a vertebrate host for the duration of their adult lives and, depending on the cestode group, the body cavity, musculature, or occasionally a diversity of other sites in one or more invertebrate and/or vertebrate hosts, as larval stages. The first larval stage in the life-cycle of a cestode is known as a decacanth embryo (in Amphilinidea and Gyrocotylidea) or hexacanth embryo (in Eucestoda); this embryo and its surrounding embryonic envelopes are collectively referred to as an oncosphere; it generally possesses either 5 (decacanth embryo) or 3 (hexacanth embryo) pairs of tiny embryonic hooks, germinative cells and two penetration glands facilitating the penetration into the body cavity of the intermediate host. In certain aquatic cestodes, the embryo is surrounded by a ciliated layer (known as a ciliated embryophore) that provides a mechanism for frees swiming; in such groups the embryo is referred to as a coracidium. The life-cycle stages that follow this stage differ dramatically among cestode groups. The terminology for these larval types has recently been reviewed and standardized by Chervy (2002). Most tapeworms have life-cycles with two hosts (dixenic life-cycle); the first one is intermediate host where the embryo developed into next larval stage; in the definitive host the tapeworm larvae attached to the intestinal mucosa and developed into mature cestode, able to reproduce. In some cestodes the accomplishment of the life-cycle needs three hosts (trixenic life-cycle) with two intermediate hosts and one definitive host.

Tapeworm body

The adult body of most tapeworms consists of an anterior scolex, often dramatically modified to assist with attachment to the vertebrate host, and a posterior strobila in which the reproductive organs are located. In polyzoic tapeworms, the strobila consists of a chain of 2 or more proglottids, each generally housing 1 set, but occasionally 2 or more sets (with a maximum of 14 sets), of male and female reproductive organs. However, in the relatively few known monozoic tapeworms, the strobila is undivided and houses a single set of reproductive organs. Some cestodes possess undivided strobila but with numerous reproductive organs; they also referred to as polyzoic cestodes, but without external segmentation. Tapeworms entirely lack a digestive system, and instead absorb nutrients through their outermost layer or tegument and its surface elaborations, referred to as microtriches (sing. microthrix). It is inappropriate to refer to the scolex of a tapeworm as its head as it lacks a mouth. Some tapeworms possess a narrow, posterior, muscular extension of the scolex known as the cephalic peduncle, which supports the scolex proper. Immediately posterior to the scolex in most cestodes is the germinative zone (also called the neck), which produces the proglottids. In some cestodes, the region posterior to the scolex is elongated and is referred to as a worm.

Scolex

Eucestodes exhibit a remarkable variety of scolex forms. Some scoleces bear, or are subdivided into, four membrane-bound, muscular structures known as acetabula. The acetabula can be in the form of bothridium, which are extensive and comprise essentially the entire scolex proper, or they can be in the form of sucker in which case, rather than comprising the entire scolex proper, they are sessile and embedded in the tissue of the scolex proper. Bothridia in some groups extend from the scolex on stalk. In other groups the left and right dorsal and ventral bothridia are each fused in back-to-back pairs, each pair being born on a pedicel. Some bothridia are subdivided into shallow depressions or loculi that are often demarcated from one another by costa; some bothridia bear hooks. In some cestodes the bothridia bear an anterior muscular pad that may or may not include an accessory sucker at its anterior margin; others bear lateral, muscular auricles or lappets. Yet other cestodes possess scoleces that consist of 2 weakly muscular bothria; some of which possess bothrial hooks. The scolex of some cestodes is divided into conspicuous anterior and posterior regions; in such cases the posterior portion is referred to as a metascolex. Cestodes possessing simple scoleces that lack bothridia or bothria and instead consist of a terminal funnel are considered to be monobothriate. The shallow depressions seen on the scoleces of some such species are also referred to as loculi.

A number of cestodes exhibit modifications of the apex of the scolex. These modifications can consist of a muscular and/or glandular apical organ. Some cestodes possess an apical organ with unarmed tentacles or 4 retractable and heavily armed tentacles. In the largest cestode order the apical organ exhibits great variability and is referred to as the rostellar apparatus. It consists of a rostellum, which is protrusible, often muscular, and in the most common case provides one or two rows of rostellar hooks. Rostellum can be withdrawn in a special muscular pouch referred to as the rostellar sheath. Ther are three main types of rostellar apparatuses: 1.) sucker-like type have muscular, cup-shaped rostellum without a rostellar sheath; 2.) davaineid type have musculo-glandular rostellum with protrusible pseuodproboscis; and 3.) sac-like type consisting of rostellum, protrusible rhynchus and rostellar sheath. Rostellum of sac-like rostellar apparatus might be retractile or invaginable, depending on the position of the rostellar hooks.

Hooks of cestodes vary significantly in their shape, morphometrics and position on the scolex. In some cestodes hooks are arranged in four pairs attached to the bothridia (bothridial hooks ). Other cestodes have large apical hooks flanked by lateral hooklets. Most cestodes possess rostellar hooks arranged in 1 or 2 crowns. In this case parts of the hooks are referred to as blade, guard and handle. In other groups hooks have only blade and basal plate. Some rostellar hooks possess epiphyseal thickenings, which may grow additionally when the worm develops within the intestine of its final host; in the intermediate hosts, larvae of such cestodes have developed only refractive particles of the hooks.

Strobila

Proglottids are produced at the posterior margin of the scolex, and thus, the youngest, or immature proglottids , are found at the anterior of the strobila, and the oldest at the posterior of the strobila. Mature proglottids ( see detail ) are those in which either both, or at least the male or female reproductive systems are fully functional. Gravid proglottids are those in which fertilization has occurred and the uterus is filled with ripe eggs. The age of the proglottids located at the posterior end of the strobila varies depending on whether proglottids detach from the posterior of the strobila when they are immature (hyperapolytic), mature (euapolytic) or gravid (apolytic), or, remain on the strobila until they senesce and eventually degenerate (anapolytic). Thus, the terminal proglottid of an anapolytic tapeworm represents the first proglottid formed, is the oldest on the strobila, and is referred to as a pygidium ; this proglottid bears the excretory pore at its posterior extremity. Alternatively, the terminal proglottid of hyperapolytic, euapolytic or apolytic tapeworms, although it is the oldest on the strobila, is most certainly not the first proglottid formed and thus the term pygidium does not apply. Proglottids are generally hermaphroditic, possessing both male and female reproductive organs. Cestodes in which the male system matures before the female system are considered to be protandrous; those in which the female system matures before the male system are progynous.

Male reproductive system

The male reproductive system consists of 1 to hundreds or, in a few cases, thousands of testes . Sperm exit each testis via a short duct or vas efferens ; the vasa efferenia from the testes unite to form a common vas deferens , which carries sperm to the terminal genitalia . The elements of the male terminal genitalia vary somewhat among cestode groups. Most cestodes possess a protrusible intromittent (copulatory) organ known as the cirrus , which is enclosedin a cirrus sac . In many cestodes the vas deferens penetrates the cirrus sac directly. In other cestodes a conspicuous dilation of the vas deferens occurs outside of the cirrus sac. This, the external seminal vesicle , serves as a temporary reservoir for sperm storage. An additional sperm storage reservoir, or accessory seminal vesicle , in the form of a sac-like diverticulum extending from the external seminal vesicle, occurs in some groups. Within the cirrus sac the duct conducting sperm only is referred to as a sperm duct (or ejaculatory duct). Some cestodes possess a dilation of this duct known as an internal seminal vesicle , which also serves as a reservoir for sperm storage. The cirrus is a modified terminal portion of the sperm duct. The cirrus may be armed with spine-like microtriches, or may lack microtriches, in which case it is considered to be unarmed . When it is everted, the cirrus releases sperm from its distal tip. In other cestodes the sperm duct unites with the vagina to form a hermaphroditic duct . In species in which this union occurs within the sac surrounding the terminal genitalia, the sac is termed a hermaphroditic sac . In some such groups the hermaphroditic duct, and not the cirrus, serves as the intromittent organ. In some groups the hermaphroditic duct is expanded to store sperm and is termed a hermaphroditic vesicle . Some cestodes have additional structures facilitating copulation. Accessory sac (or Fuhrmann's body) is sac-like, protrusible, muscular or may evert together with the cirrus during copulation. In many cestodes, the male and female ducts open into a common genital atrium and this atrium opens to the outside via a common genital pore ; in others the male and female pores open separately and are referred to as the male genital pore and female genital pore , respectively. In some cestodes the genital atrium is spherical, highly muscular and the terminal genital duct opens on the apex of a genital papilla into the atrium. The position of genital pores may vary in different cestode groups; some cestodes have median genital pores opening at the middle of the ventral (or dorsal) surface on the proglottids. In most cestodes genital pores are lateral, opening at lateral proglottid margin; in such case genital pores may be unilateral, irregularly or regularly alternating.

Female reproductive system

The female reproductive system is equally complex. Ova are generated in the ovary ; their release from the ovary into the oviduct is controlled by a sphincter-like ovicapt (or oocapt). The Mehlis' gland produces compounds involved in the formation of the egg membranes and/or egg capsules that are deposited around the egg. A vitellarium, which may be compact or arranged as a series of vitelline follicle , produces both vitelline cells to nourish developing embryos, and additional compounds involved in egg membrane and/or egg capsule formation. The vitelline products are collected and transported away from the vitellaria in 1 or more vitelline ducts that may unite to form a storage area known as the vitelline reservoir . The products of all 3 of these organs (ovary, Mehlis' gland, and vitellarium) are combined in an expanded region of the oviduct referred to as the ootype . In a few cestodes sperm are directly deposited into the ootype region by hypodermic impregnation (forceful piercing of the body wall by the cirrus). In most cestodes sperm travel from the genital pore along the vagina towards the ootype. The base of the vagina is expanded in some groups to form a sperm storage sac known as the seminal receptacle . In some species a muscular vaginal sphincter is present somewhere near the female genital opening. Sperm move from the base of the vagina into the ootype via a fertilization duct . Fertilization, packaging of the eggs with vitelline cells, and production of the egg membranes and/or egg capsules occurs in the ootype. In some groups, eggs then move via a uterine duct into the uterus , which may or may not exhibit lateral diverticula . In other groups, eggs enter 1 or more paruterine organs that lie near the uterus, in which the eggs may be packaged in saccate, thin-walled paruterine capsules that develop from the apex of the paruterine organs. In some cestodes the vagina joins the uterus forming a uterovaginal duct. Eggs are released from gravid anapolytic or apolytic proglottids either via an anterior uterine pore , from breaks or dihiscences in the body wall, or from sensing proglottids as they detach from the strobila and degenerate in bedraggled masses.

Nervous system

The nervous system of tapeworms is relatively simple. In most groups it consists of an anterior cerebral mass consisting of 2 or 4 cerebral ganglia that may be connected to one another by ventral , dorsal , and/or median commissures . Extending posteriorly from the cerebral mass are a pair of lateral longitudinal nerve cords that run the length of the worm. These primary lateral nerve cords are accompanied in some cestodes by accessory, dorsal and/or ventral lateral nerve cords. The left and right lateral longitudinal nerve cords are connected to one another via ring commissures at various points along the length of the strobila. In addition, in some cestodes anterior nerves extend forward from the cerebral ganglia and are connected to one another by an anterior ring commissure .

Excretory System

The excretory system of cestodes consists of a series of flame cells or protonephridia , nestled among the parenchymal cells of the body. These are connected to longitudinal canals that are often arranged around the perimeter of the body, or as one pair of dorsal osmoregulatory canals and a pair of ventral osmoregulatory canals . In some groups the longitudinal canals of left and right pairs are connected via transverse anastomoses ; in other groups the canals form a branched network of anastomosing osmoregulatory canals . This system functions primarily in osmoregulation, but the elimination of some excretory products may also occur. Organs located medial to the osmoregulatory canals are considered to be intervascular in position.

Other organ systems

Cestodes are acoelomates in that they lack a body cavity. The organs of their reproductive systems are surrounded by storage tissue or parenchymal cells . Directly below the tegument, they possess several very thin layers of musculature. These include circular muscles , which extend around the perimeter of the body, oblique muscles which extend perpendicular to the circular muscles, and longitudinal muscles which extend along the length of the worm. In addition, many cestodes also possess 1 or more layers of inner, longitudinal muscle bundles that extend along the length of the body from the scolex to the posterior end of the strobila. The longitudinal muscle bundles are the principle muscular system and represent the boundary between the outer cortex and the inner medulla of the body. In some species an additional layer of parenchymal muscle cells occurrs medial to the longitudinal muscle bundles.