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Life cycles of flat, round and annelids


The class Trematoda (digenetic flukes) is the largest group of parasitic flatworms. Over 6000 q have been described, and new descriptions are continually being published. There are many species that cause parasitic diseases in man and domesticated animals.

In contrast to the monogenetic trematodes, life cycles of the digenetic trematodes involve two to four hosts. The host for the adult is the definitive host, and the one to three hosts for the numerous developmental stages are termed intermediate hosts. The adhesive organs are typically two suckers. One sucker, called the oral sucker, is located around the mouth. The other sucker, the acetabulum, is located ventrally in middle or posterior end of the body.

Most digenetic trematodes are endoparasirk The definitive hosts include all groups of vertebrates, and virtually any organ system may be infected. The intermediate hosts arc largely invertibrates, commonly snails.

The life cycle is complex and will be introduced by a generalized scheme followed by more specific examples. The egg is enclosed within an oval shell with a lid, deposited in the gut, and passed to outside with the definitive host's feces. A snail may ingest an egg containing a miracidium or the ciliiated, free-swimming miracidium hatched from the egg, or the larval stage may penetrate the snail's epidermis. It thus comes to inhabit| the hemocoel.

Inside the snail the miracidium, which loses its cilia when it enters the host, begins a second developmental stage, called a sporocyst. Inside the hollow sporocyst, germinal cells give rise to a number of embryonic masses. Each mass develops into another developmental stage, called a redia or daughter sporocyst, which is also a chambered form. Germinal cells within redia again develop into a number of larvae called cercariae. The term digenetic refers to this second generation of individuals, produced asexually.

The cercaria, a fourth developmental stage, possesses a digestive tract, suckers, and a tail. The cercaria leaves the host and is free swimming. Its sec¬ond intermediate host may be an invertebrate (commonly an arthropod) or a vertebrate, in which it encysts. The encysted stage is called a metacercaria. If the host of the metacercaria is eaten by the final vertebrate host, the metacercaria escapes from its cyst, migrates, and develops into the adult form within a characteristic location in the host.

Class Cestoda (Cestoidea) Life Cycles

Tapeworms are endoparasites in the guts of vertebrates. Their life cycles require one, two, or sometimes more intermediate hosts, which are arthropods and vertebrates. The basic developmental stages are an oncosphere larva, which hatches free the egg, and a cysticercus or plerocercoid si which is terminal and develops into an adult though the following few examples illustrate the basic life cycle patterns of tapeworms, varieties exist.

Diphyllobothrium latum, one of the fish tapeworms, is widely distributed and parasitic in the gut of many carnivores, including humans. If the egg is deposited with feces in water, a ciliated, free swimming oncosphere (coracidium) hatches all an approximately ten-day development. The larva is ingested by certain copepopod crustaceans. It penetrates the intestinal wall and develops within the hemocoel into a six-hot stage called a procercoid. When the copepod is ingested by a variety of freshwater fish the procercoid, like the oncosphere, penetrates the fish's gut and eventually reaches the striated muscles of the fish to develop into a plerocercoid stage. The plerocercoid, which looks like an unsegmented tapeworm, develops into an adult tapeworm when ingested by a definitive host.

Species of the family Taeniidae are among the best known tapeworms. Taeniarhynchus saginata, the beef tapeworm, is one of the most common species in humans, where it lives in the intestine and frequently reaches a length of over 3 meters. Proglottids containing embryonated eggs are eliminated through the anus, usually with feces. If an infected person defecates in a pasture, the eggs may be eaten by grazing cattle, sheep, or goats. On hatching in the intermediate host, an oncosphere larva, bearing three pairs of hooks, bores into the intestinal wall, where it is picked up by the circulatory system and transported to striated muscle. Here the larva develops into a cysticercus stage. The cysticercus, sometimes called a bladder worm, is an oval worm about 10 mm in length, with the scolex invaginated. If raw or in-sufficiently cooked beef is ingested by humans, the cysticercus is freed, the scolex evaginates, and the larva develops into an adult worm in the gut.

Taenia solium, the pork tapeworm, is also a parasite of humans, but the intermediate host is the pig and the cysticercus is obtained from pork. Taenia pisiformis occurs in cats and dogs, with rabbits as the intermediate hosts. This order (Cyclo¬phyllidea) contains tapeworms that are largely parasitic in birds and mammals. Vertebrates, insects, mites, annelids, and mollusks serve as intermediate hosts.

A severe infection of adult tapeworms may cause diarrhea, weight loss, and reactions to the toxic wastes of the worm. The worms may be eliminated with drugs. Much more serious is cysticercus infection. Fortunately, the cysticercus stage of the beef tapeworm will not develop within humans, but this is not the case for the pork tapeworm, Taenia solium, and for the dog tapeworm, Echinococcus granulosus. The adult Echinococcus, which lives in the intestine of a dog, is minute, with only a few proglottids present at any one time. Many different mammals, including humans, can act as intermediate hosts, although herbivores are the most important in completing the life cycle. The cysticerci of the pork tapeworm develop in subcutaneous connective tissue and in the eye, brain, heart, and other organs. The bladder worm, or hydatid, of Echinococcus develops mostly in the lung or liver but can develop in many other sites as well. The bladder worms of both species can be very dangerous when growing in such places as the brain and can do much damage elsewhere. Hydatid cysts can reach a large size and contain a great vol¬ume of fluid (up to many liters), which if released into the host can cause severe reactions. Bladder worm cysts can be removed only by surgery.

Phylum Nemathelmintes


The female nematode lays eggs inside the host, which are then most often passed out in the host's feces. At this point, the eggs hatch in the external environment. Cues such as moisture levels or temperature trigger the larva inside the egg to begin producing enzymes that will dissolve the membrane of the egg. In response to the right conditions, the larva then also pushes on the weakened membrane to break through.

Stage 1 and Stage 2 Larvae

Once hatched, the nematode larvae begin to eat bacteria and grow. Some types of nematode larvae that didn't leave the initial host as an egg, such as viviparous filarial worms, are transferred to an intermediate host at this time, usually through the bite of an fly or other arthropod. Or, as in the case of Strongyloides stercoralis, they might be passed out in the feces as stage 1 larvae. When the larvae cannot grow anymore because of the size of their cuticle (skin), they must molt. To molt, they develop a new cuticle under the old one and then shed the former cuticle. The first molt marks the transition of a larva from stage 1 (L1) to stage 2 (L2).

Stage 3 Larvae

With the notable exceptions of ascarids and pinworms, most nematodes become infective during the third larval stage (L3) after the second molt. While it is during this period that they infect the host where they will eventually reproduce, different nematodes go about it in different ways. In some instances, the host may become infected by accidentally swallowing the larvae. Other stage 3 larvae, like those of the hookworm, directly invade the host. Filarial worms, on the other hand, are again transferred by fly bite from the intermediate host to the final host. Finally, Trichinela spiralis larvae remain in their initial host, go into dormancy in the muscles during stage 3, and then infect a new host when it eats the contaminated meat.


Once inside the final, or definitive, host, nematode larvae molt another two times until they develop into immature adults after four larval stages. They also migrate through the host's body, using the bloodstream or lymphatic system. The larvae travel from the heart to the lungs, trachea and intestine.


Nemotodes usually require two different sexes to reproduce and, therefore, develop into both males and females as adults. The males produce sperm in the testis and deposit them into receptacles in the female. The female holds onto the sperm until she needs to use them for fertilization. After she has finished fertilization, the eggs develop in her uterus. When they are ready, she then lays them in the host, using her muscular ovijector. At this point, the life cycle of nematodes is complete.

Phylum Annelida

Reproductive Organs

With the exception of the marine polychaetes, annelids are hermaphrodites: Each individual has male and female reproductive organs. Still, an annelid cannot reproduce without a contribution from a mate. Polychaete worms are either male or female.

Egg Laying

Earthworms, perhaps the most familiar annelids, mate before laying eggs. Two worms bind themselves to each other while each worm passes a sperm packet to the other. After mating, the broad, saddle-like band on the worm (called the clitellum) secretes a mucus sheath that begins to move toward the head of the worm. As it moves forward, the worm secretes sperm and eggs into the sheath, which eventually forms an egg cocoon. Terrestrial annelids lay their eggs in the soil, whereas aquatic annelids deposit or attach their egg cocoon to plants or to the soil substrate. Marine polychaetes transform into a reproductive stage called an epitoke before mating. The polychaete's male and female epitokes release sperm and eggs into the water.

Larval Stage

Marine polychaetes have a free-living larval stage, called a "trochophore." The trochophore eventually transforms into the adult form.

Adult Habitats

Newly hatched or metamorphosed annelids settle into adult habitats. Most adult annelids live in the soil. Marine polychaetes live in the soil substrate of their aquatic habitat. Some marine polychaetes create tubes in the mud, and these somewhat rigid tubes provide protection. Other parasitic annelids are free-living.

Adult Ecology

Most adult annelids ingest soil, digest organic nutrients and excrete the inorganic leftovers---sand particles, for example. Some parasitic species such as leeches, however, feed on other organisms. A few species even prey on other invertebrates.