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

Multicellular Animals
Flatworms
Pathogenic Cestodes - Order Cyclophyllidea

This order includes tapeworms that feature four large, rounded suckers on their scolex. The unarmed beef tapeworm (Taeniarhynchus saginatus) contains about 1,000 proglottids in its strobila and can grow up to 20 m in length (see Fig. 20).

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Fig. 20

Beef tapeworm (Taeniarhynchus saginatus):

A — general view; B — hermaphroditic proglottid; 1 — scolex; 2 — neck; 3 — strobila; 4 — bilobed Ovary; 5 — oviduct; 6 — Mehlis' gland; 7 — vitelline gland; 8 — seminal receptacle; 9 — Vagina; 10 — genital atrium; 11 — uterine pore; 12 — Uterus; 13 — copulatory organ; 14 — vas deferens; 15 — Testes; 16 — excretory canal; 17 — nerve cord.

Mature proglottids are 16-30 mm long and 5-7 mm wide. The uterus has 17-35 lateral branches. It is very difficult to expel due to its powerful suckers. The definitive host is humans, and the intermediate host is cattle. The infective larva is a cysticercus bladderworm. Humans become infected by eating poorly cooked or undercooked measly beef.

Armed pork tapeworm (Taenia solium). In addition to four suckers, the scolex also bears a double crown of hooks (Fig. 21). The strobila is more robust than that of the beef tapeworm, reaching 2-3 m in length. Mature proglottids have a uterus with 7-12 branches. Released in groups of 5-6, mature proglottids do not exhibit active movement unlike those of the beef tapeworm.

The definitive host is humans, and the intermediate host is pigs. The infective larva is a cysticercus bladderworm. Infection occurs through the consumption of unboiled or undercooked measly pork. The life cycle of the pork tapeworm is illustrated in Figure 22.

Fig. 21

General view of the pork tapeworm (Taenia solium):

1 — body of the tapeworm; 2 — scolex with suckers and rostellum with hooks.

Fig. 22

Life Cycle of the pork tapeworm (Taenia solium):

1 — mature proglottid; 2 — egg containing a larva; 3 — hexacanth larva;

4 — bladder stage (cysticercus); 5 — cysticercus with an everted scolex.

The terminal proglottids of the tapeworm are packed with developing eggs. Each of them can contain up to 175,000 eggs. Several end proglottids (usually 5-6) detach from the worm and pass out with feces, where they may be ingested by pigs. In The Stomach of the intermediate host, hexacanth larvae hatch from the eggs. They penetrate the bloodstream and are carried by Blood flow throughout the body. Most frequently, the larvae settle in the musculature and Connective Tissue, and can also reach the Liver, eyes, Brain, and other Organs. At the sites of localization, the hexacanth larvae develop into a small pea-sized cysticercus (the tapeworm scolex is inverted into this bladder). Humans contract the pork tapeworm by eating raw or undercooked meat containing cysticerci.

Another member of this order is the hydatid tapeworm (Echinococcus granulosus). It has 3-4 proglottids and is about 5 mm long (Fig. 23). Echinococcus cysticerci can reach the size of an apple and sometimes a child's HEAD. The definitive host is canids. Echinococcus undergoes the bladderworm stage in various organs (especially The Liver and Lungs) of cattle, small ruminants, pigs, and less frequently horses, rabbits, and humans. The cyst grows very slowly, taking years to develop. Cases are known where hydatid cysts weighing up to 60 kg formed in a cow's liver.

Fig. 23 Echinococcus (Echinococcus granulosus):

A — sexually mature specimen; B — structural diagram of a hydatid cyst; 1 — scolex with suckers and rostellum with hooks; 2 — immature proglottid; 3 — hermaphroditic proglottid; 4 — mature proglottid; 5 — cyst cuticle; 6 — germinative layer; 7-11 — internal daughter cysts at various Selection/3.html">Stages of development; 12-15 — external daughter cysts at various stages of development.

Another representative of the tapeworm order is the gid parasite (Multiceps multiceps). The definitive hosts of the gid tapeworm are members of the canine family, most commonly dogs. Sheep serve as intermediate hosts. They become infected by ingesting the eggs of the gid parasite along with grass contaminated by the feces of infected dogs. The infective larva is a coenurus, which develops in one of the cerebral hemispheres of sheep, causing a dangerous disease known as "gid" or coenurosis.

Prevention of tapeworm infections includes washing hands and vegetables, boiling Water from open water bodies, avoiding chewing on blades of grass, thoroughly cooking or roasting meat and fish, burning measly meat, implementing public sanitation measures to prevent animal infection with worm eggs, identifying and treating infected individuals, and carrying out educational Sanitary and hygienic outreach, etc.

Phylogeny of the phylum Platyhelminthes and the question of THE ORIGIN OF parasitism

Flatworms share an evolutionary origin with primitive Coelenterates. At the Base of the Plathelminthes Phylogenetic Tree undoubtedly lies the class of turbellarians (Turbellaria).

The transition of free-living animals to parasitism can occur through several pathways. Of great importance in this regard are various types of Symbiosis, including so-called “commensalism,” where a smaller partner finds shelter on the body of a larger host. Such commensals may well have been the turbellarian-like ancestors of modern monogenean flukes.

Settling On the surface of the gills, Skin, or fins of fish, they initially fed on small invertebrates settling in the same areas or on mucus secreted by the fish's integument. It is likely that instead of being content with this intermittently available food source, such commensals began to inflict wounds on the host's body and thereby discovered an inexhaustible supply of nutrients. Having gradually transitioned to feeding on the host's blood and Tissues, they thus evolved into true ectoparasites.

In certain cases, ectoparasitism can subsequently lead to internal parasitism. For instance, many monogenean flukes spend their entire lives on the gills of fish.

Some species of modern monogeneans parasitize the Oral Cavity, Pharynx, and Esophagus of fish. Evidently, the course of evolution involved a gradual migration of worms—originally gill parasites—into the Oral Cavity and further along the digestive tract. Phylogenetically, this represents a definitive pathway of transition from ectoparasitism to intestinal parasitism within the host. It was precisely in this manner that the classes of cestodes and cestodiforms could have arisen from ancient monogenean-like ancestors.

The phylogenetic Development of the modern class of trematodes followed a somewhat different path, although symbiotic relationships also formed The basis of their adaptation to parasitism. The life cycle of modern digenetic flukes cannot be completed without the obligatory involvement of Mollusks. It is probable that mollusks were the phylogenetically primary hosts of trematodes. It is easy to envision The Emergence of symbiosis between mollusks and the free-living ancestors of digenetic flukes.

The larvae of the latter, resembling rhabdocoel turbellarians, likely inhabited the bottom zone near stones or utilized mollusk shells as shelter. In doing so, they could easily enter the gill cavity of mollusks and settle there, thus becoming commensals. Naturally, such a transition from a free-living existence to commensalism must have occurred very gradually, with the larvae maintaining a connection to the external environment for a prolonged period.

Subsequently, these commensal forms—initially feeding perhaps independently of the mollusks—could have switched to consuming their blood, thus embarking on the path of adaptation to parasitism. Gradually, this would have led to the migration of the parasites into the interior of the mollusk's body. Shielded by the host Organism from adverse environmental conditions, they simultaneously gained a constant food source in the form of host fluids and tissues. This drove a new direction in the evolution of trematode ancestors, manifested in the progressive and increasingly profound simplification of their body Organization and the acceleration of maturation.

Larvae that previously left their host and attained sexual maturity in the external environment now ceased to emerge and began reproducing within the host's body. Since the ancestors of trematodes retained a free-living generation as well, their life cycle acquired the character of heterogony. The difference, however, lies in the fact that individuals of the parasitic generation underwent simplification under METABOLISM/18.html">The Influence of parasitism, transforming from hermaphrodites into parthenogenetic females.

QUESTIONS FOR SELF-check and review:

1. What general features characterize the Representatives of the phylum flatworms?

2. What are the biological characteristics of flukes?

3. What is the developmental cycle of the liver fluke?

4. What is the MORPHOLOGICAL Structure OF tapeworms?

5. What main stages characterize the life cycles of tapeworms?

6. Which tapeworms are parasites of humans and animals?

7. What are the preventive measures against tapeworm infection?