Which classes of platyhelminthes are parasitic

The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex life cycles that involve a primary host in which sexual reproduction occurs and one or more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk. Trematodes are responsible for serious human diseases including schistosomiasis, a blood fluke.

The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates. Tapeworms live in the intestinal tract of the primary host, remaining fixed by using a sucker on the anterior end, or scolex, of the tapeworm body. The remainder of the tapeworm is composed of a long series of units called proglottids.

Each may contain an excretory system with flame cells and both female and male reproductive structures. Learning Objectives Differentiate among the classes of platyhelminthes. Key Points The Platyhelminthes are acoelomate flatworms: their bodies are solid between the outer surface and the cavity of the digestive system.

Most flatworms have a gastrovascular cavity rather than a complete digestive system; the same cavity used to bring in food is used to expel waste materials. Platyhelminthes are either predators or scavengers; many are parasites that feed on the tissues of their hosts. Flatworms posses a simple nervous system, no circulatory or respiratory system, and most produce both eggs and sperm, with internal fertilization.

Platyhelminthes are divided into four classes: Turbellaria, free-living marine species; Monogenea, ectoparasites of fish; Trematoda, internal parasites of humans and other species; and Cestoda tapeworms , which are internal parasites of many vertebrates.

In flatworms, digested materials are taken into the cells of the gut lining by phagocytosis, rather than being processed internally. Key Terms acoelomate : any animal without a coelom, or body cavity ectoparasite : a parasite that lives on the surface of a host organism scolex : the structure at the rear end of a tapeworm which, in the adult, has suckers and hooks by which it attaches itself to a host proglottid : any of the segments of a tapeworm; they contain both male and female reproductive organs.

Excretion and osmoregulation by flatworms is controlled by "flame cells" located in protonephridia these are absent in some forms. Flatworms lack a respiratory or circulatory system; these functions take place by absorption through the body wall. Nonparasitic forms have a simple, incomplete gut; even this is lacking in many parasitic species. Movement in some flatworms is controlled by longitudinal, circular, and oblique layers of muscle.

Others move along slime trails by the beating of epidermal cilia. The development of directional movement is correlated with cephalization. In some flatworms, the process of cephalization has included the development in the head region of light-sensitive organs called ocelli.

Other sense organs found in at least some members of this group not necessarily on the head include chemoreceptors, balance receptors statocysts , and receptors that sense water movement rheoreceptors. Most flatworms can reproduce sexually or asexually. Most are monoecious. Most of these have developed ways of avoiding self-fertilization. Excretory tubes lead to a common channel, which has two body openings. The smooth, gliding motion is achieved in part by the beating of cilia and in part by muscular contractions of the body wall.

Eye spots on the animal are a shade that overhangs light-sensitive cells. Auricles are sensitive to chemical stimulation and aids in the location of food. The whole body surface also is sensitive to touch and chemical stimuli. Nervous System. Various nerve processes extend off of the nerve cords. This is the first nervous system with a coordinating ganglion and the first to show nerve cords.

The sexual reproduction is the best developed of all systems. The animals become sexually mature only at rare intervals, which is triggered by conditions in the environment. They are hermaphroditic but cross-fertile. Any one animal can act as male or female, but only one sex at a time.

The male sexual structures consist of testes, vasa efferentia 2 , vasa deferentia 2 , a penis and a genital pore. The female sexual structures consist of ovaries, an oviduct, a vagina, a uterus, a bursa and a genital pore. The genital pore is common to both male and female. Eggs are laid in a cocoon and vitellaria , or yolk glands , are present. These glands secrete yolk in the form of actual cells. The yolk cells are collected down in the vitellaria and serve to provide food for the developing embryo.

The eggs are ectolecithal as the yolk occurs on the outside of the egg. During asexual reproduction the posterior part of the animal simply breaks off. This is the most common means of reproduction in Dugesia. There is a polarity that exists in each cut piece. The edge that was closest to the anterior end of the animal will develop a new head, and that closest to the posterior end will develop a new tail. Sections cut from the center of the animal will develop into a whole new animal.

Economic Importance. Review of The Turbellaria. There are many variations in the form of the digestive tract and the position of the mouth. Many species possess suckers. Sexual reproduction is found in all, but asexual reproduction is the most common in the group.

In Rhabdocoela there are nematocysts, which are acquired from consuming Cnidiarians. Most Turbellaria have direct development. Eggs are held in a cocoon from which adults hatch. A free-swimming larval stage does occur, which is known as Muller's Larva. The Class Trematoda include the flukes, and two Subclasses will be presented here: Monogenea and Digenea. Monogenea is a small group whose members are external parasites on the gills of fish, in the mouth cavity, etc.

There is a direct development without a larval stage or intermediate host. Digenea are internal parasites, mostly on vertebrates. There is an indirect development where larval stages are living in intermediate hosts. The life cycles of the Digenea are quite complex. All species of Trematoda are parasitic but they are mostly internal parasites, with fewer numbers being external parasites.

They are generally similar in structure to the Turbellaria but with several modifications. They have more complicated life cycles that involve several host animals. A representative Genus is Pneumonoeces , which includes the lung flukes of frogs and toads. The adults of these animals live in the lungs of frogs and toads, while the larvae are parasitic on snails and dragonflies.

There is also a sucker on the ventral surface called the acetabulum that has no opening. They have very poorly developed muscle layers. The mesenchyme bears cells, which secrete the cuticle. The cuticle protects the fluke from the host's digestive fluids. There are no cilia, hence no locomotion!

A muscular pharynx located behind the oral sucker serves as a food pump. There is a single-branched intestinal tract that has no subdivisions. An exception is in the sheep liver fluke. Muscle layers are weakly developed. Sensory cells are generally diminished as compared to the Turbellaria as there are little external stimuli that reach these animals due to their interior environment.

In sexual reproduction the animal is hermaphroditic and is self or cross-fertile. Males have two testes and a cirrus , which turns inside out as it is protruded. It is the expanded, eversible end of the vas deferens. The base of the cirrus stores sperm and is the seminal vesicle.

The female has one ovary, and the uterus winds around and empties out at the top of the cirrus sac. A shell plus about 12 yolk cells are put around the eggs. The eggs are then laid in tremendous numbers and already contain embryos at the laying time.

Asexual reproduction is found in the life cycle in different hosts. There are such structures as germ bulbs, rediae, etc. Life Cycle. The miracidium larval stage then commences. It is covered with cilia and possesses a pair of eyes and an epidermis. It swims around in the water until it finds a certain kind of snail. It then bores through the snail body and turns into a sporocyst.

The sporocyst develops germ bulbs within its body, each bulb developing into the next larval stage, the redia. The redia taken on more characteristics of the fluke.