Cestodes

Cestodes, or tapeworms, Cestoda Gegenbaur, 1859 (syn. Cestoidea Paid., 1808, Cestodes Burmeister, 1837, Cysticerci Zeder, 1800, Taeniada Nicholson, 1887), constitute a class of the flatworms type (Platodes Leuck. 1854) and are without exception parasitic organisms. - Anatomical-morphological characteristics. Most C. have a ribbon-like body, the so-called strobila, consisting of a head--scolex, an unsegmented neck, and a greater or lesser number of individual segments-proglottids (from one to several thousand). The size of tapeworms varies within very wide limits: from fractions of a millimeter to 10 m or more. The scolex serves as the organ with which the parasite attaches to the host's tissues; corresponding to this function, the scolex is armed with special elements: hooks, suckers. The latter are represented in the form of semicircular suckers (acetabula), either replacing them with bothridia, which are more or less flat, elongated-oval folded elements sitting directly on the scolex or connected to it by means of a stalked pedicle, or finally in the form of sucking pits-bothria. In many C., the scolex has chitinous hooks, often located on a special rostellum of the scolex. Behind the scolex, the body of C. usually narrows slightly, forming a neck, which performs the following physiological role: from it the segments grow, so that the youngest segment will always be near the neck, and the old segments are always pushed back and the last segment will always be the oldest. This process of growth of tapeworms is called strobilation. The shape of proglottids varies depending not only on the species of worm but also on the age of the segment (most often quadrangular, somewhat elongated in the longitudinal or transverse direction). Proglottids are filled mainly with sexual elements, and each segment contains its own system of organs, developing in a certain sequence: first, the male sexual organs appear, then the female sexual organs are laid down. Thus, cestodes are hermaphrodites. The exception is the single genus Dioicocoestus, which is a dioecious organism. In a segment that has reached the hermaphroditic stage, with further growth, the male sexual elements-seminal vesicles-first begin to undergo reduction; then the female sexual organs gradually disappear, except for the uterus, which, filling with eggs, constantly increases in volume and usually occupies almost all the internal space of the segment. Thus, the most posterior, mature segments of the strobila are female, consisting of a uterus containing eggs. The body of C. is covered with a cuticle, under which lie thin muscles, and the spaces between the organs of the sexual system are filled with the so-called parenchyma. The nervous system consists of a central nerve node located in the scolex, and longitudinal trunks extending from the node, which pass through the entire length of the parasite and are connected to each other by transverse branches. The excretory system is of the trematode type. .Tapeworms have no digestive system: they feed on food masses of their host, absorbing them with the entire surface of the body.

Figure 1. Cestodes of the order Tetraphyllidea: a-scolex of a representative of the genus Acanthobothrium; b-hermaphroditic segment of Acanthobothrium ijimai. (After Furman, 1931.)

The sexual system of C. is very developed, fills almost the entire segment, and is basically built as in trematodes. A specific feature of C. is the presence of a vagina in the female organs system. It is a muscular canal that is usually located next to the vas deferens, opening

Figure 2. Cestodes of the order Diphyllidea: a-scolex of Echinobothrium affine with a neck armed with rows of T-shaped hooks; b-hermaphroditic segment of the same species. (After Furman, 1931.)

next to the male sexual opening. Another, inner end of the vagina connects with the oviduct; in this part, the vaginal canal usually has an expansion, which is called the seminal receptacle, where spermatozoa that have entered the vagina during copulation of C. are preserved. The duct of the vitelline gland and the ducts of the Mehlis' gland also approach this place. This place, as in trematodes, is called the ootype. It is here that fertilization of eggs occurs, here the eggs are supplied with the yolk material necessary for the nutrition of the embryo, are covered with a shell and move into the uterus, - a special organ in which the eggs are stored until the moment when they will be expelled into the external environment for the further development of the embryos in the body of the intermediate host. The uterus in C. is structured differently. There is a special group

Figure 3. Cestodes of the order Tetrarhynchidea: a-scolex of a representative of the genus Tetrarhynchus with four retractable rostella; b-structure of the hermaphroditic segment of the same parasite. (After Furman, 1931.)

of C., the tapeworms, in which the uterus is arranged in the form of a looped canal and has an opening for the exit of eggs, i.e., the uterus is open. In these cases, mature eggs exit the uterus into the host's intestine and are excreted outward with feces. In most C. parasitic in humans (tapeworms), the uterus has the form of a blind sac, without an exit opening, i.e., the uterus is closed, due to which the eggs of these parasites are not excreted into the host's intestine with feces, but exit outward by detaching mature segments from the strobila. As a result, tapeworms have three sexual openings: one male and two female-vagina and uterus; in tapeworms, two: male and female-vagina. There also exist in nature such tapeworms in which each segment has two sets of sexual systems, and either all sexual glands, except the uterus, can be duplicated, or the uterus is duplicated together with all glands and the uterus. - The eggs of C. have diverse structures. In the most primitive groups, the one-segmented C. (Cestodaria), the eggs are built the same as in trematodes, with the difference that the larva contained in the eggs (or developing in them) is equipped with 10 hooks and is called a oncosphere. The eggs of higher C. (tapeworms) have a delicate outer ^aesH&^/fSflBafcft shell, sometimes provided with filaments; inside this shell is an embryo, the so-called oncosphere, surrounded by a special em

_ _tt Yatlttta TTG.GTL» _pйп ^Ris- ⁵- Cestodes of the order Cyclophyllidea: a - scolex with two suckers. Onchosphere of Dipylidium caninum; b - hermaphrodite represents a mature segment of the same parasite, a spherical or oval-shaped embryo, equipped with three pairs of hooks. In tapeworms, the eggs are similar to those of trematodes, but the embryo developing in them also has three pairs of hooks. Systematics of cestodes. C. are conventionally divided into 6 orders (fig. 1-5): 1) Cestodaria - strobila from one segment with a single sexual apparatus. Embryo of the lophophore type with 10 hooks. 2) Diphyllidea - multi-segmented C.; scolex equipped with 2 bothridia and a long neck covered with numerous T-shaped hooks. Uterus closed. Cirrus and vagina open ventrally. 3) Tetrarhynchidea - multi-segmented C. with a scolex equipped with 2-4 bothridia and 4 retractable hooks on the rostellum. Uterus closed or opens ventrally. 4) Tetraphyllidea - multi-segmented C. with a scolex equipped with: 4 bothridia or round suckers. Uterus closed. Vitellaria - along the lateral edges of proglottids. 5) Pseudophyllidea - see Tapeworms. 6) Cyclophyllidea - see Tapeworms.-Biol. cycle of C. occurs through the change of hosts, with some C. having a change of two hosts (tapeworms), while others (tapeworms) involve three hosts. Definitive hosts of C. (with rare exceptions) are vertebrates, intermediate hosts are the most diverse animals, including arthropods, and additional hosts are various vertebrates (fish, amphibians, reptiles, mammals, birds). Mature C. are localized mainly in the intestinal lumen, less frequently in the abdominal cavity and in the liver. In relation to hosts, mature C. are quite stenoadaptive, affecting only closely related animal species, but there are exceptions, e.g. Dipylidium caninum L. (see) and others. In the larval stage, C. are generally less specific in relation to hosts; for example, Taenia solium, which in the mature stage infects only humans, in the larval stage can inhabit representatives of various orders of mammals or Echinococcus granulosus, the strobila of which can only inhabit carnivores, while the larvae inhabit a huge number of mammal species. Types of larval cestode structure. Several main forms of larval C. are distinguished, mostly characterized by fluid-filled blisters. The wall of the blister usually consists of two layers - outer and inner. On the inner wall of the blisters, the scoleces of future parasites are formed. 1) A blister in which only one scolex is formed is called a cysticercus (see). 2) There are species of C. whose larvae also represent blisters, with many (dozens, even hundreds) scoleces forming on the inner membrane of the blister. This type of larva is called a cenurus and causes an invasion called cenurosis (see). 3) The most complex blister is the larva called echinococcus (see). 4) If the larva does not have the character of a blister, but consists of an anterior swollen part and a posterior part resembling a tail appendage in which embryonic hooks are located, then such a larva is called a cysticercoid. This type of larva is characteristic of Hymenolepis nana (see Hymenolepis) and Dipylidium caninum L. (see). 5) Plerocercoid is the name given to the larva of tapeworms, located in the body of an additional host and consisting of a small unsegmented strobila with bothridia at the head end. Phylogeny of cestodes. The former assumption about the origin of C. from trematodes is not supported by recent authors. It should be assumed that C., like trematodes, originated from turbellarians Rhabdocoela, in particular from parasitic Anoplodiidae. Furman considers that the most primitive group of C. are Tetraphyllidea, parasitizing in the most ancient vertebrate cartilaginous fishes (Selachii); from Tetraphyllidea, on the one hand, Tetrarhynchidea (mainly parasites of cartilaginous fishes) and Pseudophyllidea (parasites of marine bony fishes - Teleostei, freshwater fishes and terrestrial vertebrates) originated; on the other hand, Tetraphyllidea gave rise (through the family Proteocephalidae) to the order Cyclophyllidea, parasites mainly of higher animals - birds and mammals, including humans.