Cestodes

CESTODES, or tapeworms, Cestoda Gegenbaur, 1859 (syn. Cestoidea Rud., 1808, Cestodes Burmeister, 1837, Cysticerci Zeder, 1800, Taeniada Nicholson, 1887), constitute a class of the flatworm type (Platodes Leuck. 1854) and are without a single exception parasitic organisms. - Anatomo-morphological characteristics. The majority of cestodes have a ribbon-like body, the so-called strobila, consisting of a head-scolex, an unsegmented neck, and a greater or smaller number of individual segments-proglottids (from one to several thousand). The size of tapeworms ranges within very wide limits: from fractions of a millimeter to 10 m and more. The scolex serves as an organ by means of which the parasite is fixed to the host's tissues; in accordance with this function, the scolex is armed with special elements: hooks, attachment organs. The latter are presented in the form of semicircular suckers (acetabula), or bothridia replacing them, which are more or less flat, elongated-oval folded elements sitting directly on the scolex or connected to it by means of a stalked leg, or finally in the form of attachment pits-bothria (bothria). In many cestodes, there are chitinous hooks on the scolex, often located on a special proboscis (rostellum) of the scolex. Behind the scolex, the body of the cestode usually narrows slightly, forming a neck, which performs the following physiological role: the growth of segments originates from it, so that the youngest segment will always be near the neck, and the old segments are all pushed back and the last segment will always be the oldest. Such a process of tapeworm growth is called strobilation. The shape of the proglottids varies depending not only on the species of the 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 reproductive elements, and each segment encloses an independent system of organs developing in a certain sequence: first, parts of the male reproductive apparatus appear, then the female reproductive organs are laid down. Thus, cestodes are hermaphrodites. An exception is the single genus Dioicocestus, which is a dioecious organism. In a segment that has reached the hermaphroditic stage, in the process of further growth, first of all, the male reproductive elements-testicles-begin to undergo reduction; then the female reproductive organs gradually disappear, with the exception of the uterus, which, filling with eggs, continues to increase in volume and usually occupies almost the entire 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 the cestode is covered with a cuticle, under which thin muscles are laid, and the spaces between the organs of the reproductive system are filled with the so-called parenchyma. The nervous system consists of a central nerve ganglion located in the scolex, and longitudinal trunks extending from the ganglion, which pass through the entire ribbon of the parasite and are connected to each other by transverse branches. The excretory system is of the trematode type. There is no digestive system in tapeworms: they feed on the food masses of their host, absorbing them through 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 Fuhrmann, 1931.)

The reproductive system of cestodes is very lushly developed, fills almost the entire segment, and is built basically like that of trematodes. A specific feature of cestodes is the presence of a vagina in the female organ system. It is a muscular channel 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 Fuhrmann, 1931.)

next to the male genital opening. The other, internal end of the vagina connects to the oviduct; in this part, the vaginal canal usually has an expansion, which is called the seminal receptacle, where spermatozoa that penetrated into the vagina during cestode copulation are stored. The outlet channel of the vitelline glands and the ducts of the glands of the Mehlis body approach this same place. This place, as in trematodes, is called the ootype. It is here that the fertilization of eggs takes place, and here

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

eggs are supplied with yolk material necessary for the nutrition of the embryo, covered with a shell, and advanced into the uterus—a special organ in which the eggs are stored until the moment when they are carried out into the external environment for the further development of embryos in the body of the intermediate host. The uterus in cestodes is constructed differently. There is a special group

Figure 4. Cestodes of the order of broad tapeworms-Pseudophyllidea: a-scolex of a representative of the genus Diphyllobothrium; b-hermaphroditic segment of the same parasite. (After Fuhrmann, 1931.)

of cestodes, the pseudophyllideans, in which the uterus is built in the form of a convoluted channel and has an opening for the exit of eggs, i.e., an open uterus. In these cases, mature eggs leave the uterus into the host's intestine and are excreted with feces. In the majority of cestodes parasitizing humans (taeniids), the uterus has the shape of a blind sac, without an exit opening, i.e., a closed uterus, due to which the eggs of these parasites are not excreted into the host's intestine with feces, but come out by the detachment of mature segments from the strobila. As a result, pseudophyllideans have three genital openings: one male and two female—of the vagina and uterus; taeniids have two: male and female—of the vagina. There are also such tapeworms in nature in which each segment has two assortments of the reproductive system, and either all reproductive glands except the uterus can double, or the uterus doubles along with all the glands. - Cestode eggs have a diverse structure. In the most primitive groups, single-segmented cestodes (Cestodaria), eggs are built in the same way as in trematodes, with the difference that the larva contained in the eggs (or developing in them) is provided with 10 hooks and is called a lycophore. Eggs of higher cestodes (taeniids) have a delicate outer membrane, sometimes provided with filaments; inside this membrane is enclosed the embryo, the so-called oncosphere, surrounded by a special embryophore.

Fig. 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.