Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Flatworms
Class Cestoda

There are approximately 3,400 known species of tapeworms. All cestodes lead a parasitic lifestyle. In their sexually mature stage, they inhabit the bodies of fish, amphibians, reptiles, and mammals, including humans. Their larvae, meanwhile, settle in various invertebrates. Those Cestoda species whose larvae, like the adults, live in the bodies of vertebrates pose the greatest threat to humans. Cestodes typically localize in the definitive host (vertebrate animals and humans) within the intestinal lumen, and occasionally in The Stomach or Liver ducts.

One of the most Characteristic Features of cestodes, as a highly specialized group of parasitic organisms, is the complete absence of digestive Organs. Nutrient absorption is carried out directly through their body surface. The extraordinarily high fecundity of this group of worms is associated with the strong development of their Reproductive System and Hermaphroditism. Another feature that has emerged in The process of tapeworm adaptation to parasitism is The complexity of their life cycles. Thirty-two species of cestodes parasitize humans, over 30 parasitize cattle, and even more parasitize sheep and goats. Diseases caused by cestodes are referred to as cestodiasis.

Structure. The body is long and ribbon-like. It is divided into 3 regions: the HEAD (scolex), the neck, and the strobila, which consists of numerous segments (proglottids).

The head bears attachment organs — suckers, chitinized hooks, and bothria. Bothria are characteristic of tapeworms (order Bothriocephalidea). More highly organized tapeworms are equipped with suckers (usually 4 in number). A head equipped with suckers may also possess a rostellum, which is frequently armed with hooks. The structure of the head, particularly the attachment organs, varies considerably and therefore serves as a systematic taxonomic feature.

The neck is an unsegmented region of the body that connects the head to the first body segment. The youngest segments originate immediately behind the neck, while the older ones are located toward the posterior end of the body. The number of segments can range from tens and hundreds to several thousand in some species. Consequently, the length of the cestode body can vary from a few millimeters to 20 m.

The Nervous system is poorly developed, consisting of a central ganglion located in the head and 2 nerve cords extending from it along the length of the body. Sense Organs are represented by sensory Cells scattered throughout the body.

The excretory system is of the protonephridial type. There are 2 main canals running the entire length of the body, which originate at the posterior end. Toward the anterior end, they form loops and turn backward, effectively forming 4 canals. In segmented cestodes, the lateral canals are connected by transverse anastomoses in a ladder-like pattern, both in the head and near the posterior margins of the segments. A common bladder is initially present at the junction of the 2 canals. However, when the terminal segment detaches, a new bladder is no longer formed, and each lateral canal opens to the exterior via a separate pore.

Reproductive system. Tapeworms are hermaphrodites. Reproductive organs are located within each segment and are not connected to the reproductive organs of adjacent segments. A sequential Development of the male and female reproductive systems is characteristic. Typically, Male reproductive organs develop first, followed by The formation of female organs. After Fertilization, the male system gradually degenerates, while the Uterus develops intensively as eggs are produced. The terminal segments are mature, almost entirely filled with the uterus, and contain numerous eggs. Eggs are released into the external environment when the walls of a segment that has detached from the strobila rupture.

Individual Components of the reproductive apparatus vary quite significantly among different tapeworms; therefore, for the sake of concreteness, we will examine a specific case, such as the unarmed beef tapeworm (*Taeniarhynchus saginatus*).

In the young anterior segments of the strobila, the reproductive organs are not yet developed and begin to form only around the 200th segment. The Male Reproductive System is represented by numerous Testes scattered throughout the parenchyma. The fine vas efferentia of the testes unite to form a common vas deferens, which extends toward one of the narrow lateral margins and there penetrates the copulatory organ—a muscular tube whose tip projects into the genital atrium.

The FEMALE REPRODUCTIVE SYSTEM consists of a branched Ovary, an oviduct, and an ootype. The Contents of the unpaired vitelline gland also empty into the ootype. In addition, 2 canals depart from the ootype. One is the Vagina, which runs alongside the vas deferens and opens adjacent to it into the genital atrium. The other is a wider canal that extends from the ootype along the median line of the segment and ends blindly; this is the uterus.

Oocytes enter the ootype, where sperm cells also penetrate via the vagina. Here the eggs are fertilized, surrounded by a shell, and transferred to the uterus, where they undergo the first part of their development. The eggs crowd the uterus to such an extent that the latter expands significantly, gives rise to numerous lateral branches, and occupies a major portion of the segment. By this time, all other PARTS OF THE reproductive system have completed their function and undergone greater or lesser atrophy.

Segments containing only a greatly enlarged and branched uterus packed with eggs are called mature segments. They occupy the posterior end of the strobila and periodically detach in groups. The largest cestodes produce a staggering number of eggs over their long lifespans. For example, *Taeniarhynchus saginatus* lives for 18–20 years and produces up to 600 million eggs per year, totaling approximately 11 billion over its lifetime.

Other cestodes may exhibit a variety of deviations. For instance, while the uterus is blindly closed in tapeworms of the family Taeniidae, in broad tapeworms such as *Diphyllobothrium latum*, it opens to the exterior on one of the flat surfaces of the segment. In such species, as eggs accumulate in the uterus, they pass out into the lumen of the host's intestine. A curious modification observed in some forms is the partial or complete duplication of the reproductive apparatus in each segment (for example, in the pumpkin seed tapeworm, *Dipylidium caninum*).

Reproduction and development. Fertilization in tapeworms occurs both through cross-fertilization and self-fertilization.

We will examine The life cycle first using the specific example of the pork tapeworm (*Taenia solium*), which parasitizes humans in its adult stage. Eggs are passed out with feces, either through the rupture of segment walls or along with detached segments. For further development, the eggs must reach a specific intermediate host, which for *Taenia solium* is the pig. Pigs become infected due to lack of hygiene.

Once the eggs reach the pig's intestine, their shell dissolves, and a larva known as an oncosphere emerges.

The oncosphere is a hexacanth embryo—a small multicellular sphere equipped with 6 chitinized hooks. During subsequent development, these hooks are eventually shed. Using these hooks, the oncosphere bores through the stomach or intestinal wall, enters the lymphatic or Blood Vessels, and is carried by the bloodstream to various Internal Organs—most frequently The Liver and Muscles, and less commonly the Lungs, Brain, and others. Here the oncosphere becomes lodged and transforms into a bladder worm (cysticercus or finn).

The body of the larva becomes hollow, forming a fluid-filled bladder. An invaginating bud develops—the primordium of the head, though inverted inward within the bladder. No further development of the finn occurs within the pig's body, but it can survive there for several years without perishing.

To achieve sexual maturity, the finn must reach the intestine of the definitive host, which is a human. There, the finn is freed from the surrounding meat under METABOLISM/18.html">The Influence of digestive juices, and subsequently the head of the finn everts outward (primarily under the influence of Bile), allowing the hooks and suckers to assume their normal functional position. The Head and Neck begin to grow intensively and bud off new segments at the posterior end of the body, forming a long chain of proglottids—the strobila.

In other cestodes, the general course of development is similar, but certain details—some quite significant—may vary. For instance, in the broad fish tapeworm, development involves not one, but two intermediate hosts. The eggs must reach Water, where they hatch into a ciliated, hexacanth larva known as a coracidium. It swims about, is ingested by a cyclops copepod, and inside the copepod, the coracidium transforms into a procercoid—a small, worm-like larva (0.5 cm). The Development of the procercoid takes about 3 weeks.

Fish become secondary intermediate hosts by feeding on infected copepods. Within them, a plerocercoid (1.0–1.5 cm) is formed. This stage corresponds to the finn stage and represents the infective larva. The development of the broad fish tapeworm is completed in the intestine of a human, dog, or cat, which serve as the primary, i.e., definitive hosts.

In addition to the plerocercoid and the pork tapeworm finn described above, Other types of larval bladder worms (finns) also exist.

1. Cysticercus — a small hollow bladder containing a single invaginated head.

2. Coenurus — a specific type of finn in which the bladder enlarges considerably and, instead of a single invagination, develops numerous invaginations on its wall, thus giving rise to multiple heads.

3. Echinococcus — the bladder grows to an enormous size, developing numerous secondary smaller bladders (brood capsules) both internally and along its walls. Each brood capsule is functionally equivalent to a single coenurus, meaning that a single hexacanth embryo gives rise to several thousand scolices.

Despite their external differences, all of the aforementioned bladderworm forms are variants of a single underlying type.