introduction of platyhelminthes

Phylum Platyhelminthes

Phylum Trend Synopsis: Commonly called flatworms, the phylum name is derived from Greek  and describes the body shape of these animals: dorsally ventrally flattened but otherwise worm-like. The approximately 20,000 species are classified into four classes: the free living flatworms in the Class Turbellaria, the parasitic monogeans in the Class Monogea, the parasitic flukes in the the Class Trematoda, and the parasitic tapeworms in the Class Cestoidea. The body plan is that of an elongated organism that is dorsally ventrally flattened with bilateral symmetry and with a hint of cephalization. Distinct tissues and organ systems are found, but the organs are not located in a body cavity. Cross sections of these animals show that, except for the digestive cavity, no other internal cavity is found. Because the organs are in direct contact with a surrounding loosely organized tissue, these animals are said to be acoelomate. These animals are saoid to be triplpoblastic because three layers of cells can be seen: an outer epidermis (ectoderm), a middle layer that may be several cells thick (mesoderm)and a distinct lining of the gut the endoderm. The digestive system is described as incomplete because it is not a tube with a mouth at one end and anus at the other. There is only one opening, so it is described asa sac-like system. Food enters the mouth and is digested in a branched gastrovascular cavity. Any non-digestable residue must be regurgitated back through the mouth. Muscle layers are well developed and controlled by a distinct nervous system with two longitudinal nerve cords that run the length of the body. Two nerve ganglia and sensory receptors at the anterior end coordinate activity sending nerve signals to the rest of the body by two  ventral longitudinal nerve cords. There is no separate skeletal system and body support is by hydrostatic pressure or by a cuticle. There are no special organs for respiration and circulation. Diffusion across the body surface or from the gastrovascular cavity to the body cells fulfills these functional needs. Individuals may have both male and female sex organs, producing sperm and eggs. Fertilization is internal.  Microscopic fertilized eggs are released. Development may be direct into minature adult forms or indirect through a indepemdent larval stage. As parasites of animals, the platyhelminthes have substantial impacts on ecosystems.

Modern analysis of DNA sequences indicates that these animals may be related to the protostome branch of the animal phylogentic tree. They are placed in a newly defined clade called the Lophotrochozoa along with rotifers, annelids, and molluscs.  This new grouping suggests that the lack of a body cavity may not be a primitive condition but one that evolved secondarily (later) from animals having a coelom.

Class Turbellaria

 

Class Turbellaria has approximately 3000 species in 12 orders. Most are marine and benthic, but some also inhabit fresh water and moist temperate and tropical terrestrial habitats. The order Temnocephalida is entirely commensal or parasitic, but some members of other orders are commensal as well. Classification within the Turbellaria is in a state of flux as it is unclear whether the class is truly monophyletic. Currently orders are defined by the type of pharynx, organization of the reproductive system, and branching pattern of the gut. Orders of the class Turbellaria recognized by Ruppert and Barnes (1994) are: Nemertodermatida, Acoela, Catenulida, Haplopharyngida, Macrostomida, Polycladida, Lecithoepitheliata, Prolecithophora, Proseriata, Temnocephalida, Rhabdocoela, and Tricaldida. Brusca and Brusca (2003) include Temnocephalida as a suborder of Rhabdocoela, and recognize an additional class, Proplicastomata, which includes known specimens from Greenland.

Like other members of the phylum Platyhelminthes (flatworms), those of the class Turbellaria are dorsoventrally compressed, with high surface area to volume ratios. Marine species can be quite colorful, but the interstitial and terrestrial turbellarians tend to be drab. Turbellarians generally locomote by coordinated waves of cilia on a secreted mucus trail, though some species can swim by rhythmic muscle contractions. Their ciliated epidermis, the presence of sub-epidermal rhabdites, and their free-living condition distinguish turbellarians from members of the other classes of Platyhelminthes. The visual impression of their epidermal ciliary activity gives this group its name, as 'Turbellaria' is derived from the Latin term for "whirlpool."

Turbellarians lack fluid transport systems, and are acoelomate. The gut in turbellarians has only a mouth opening. There is usually a pharynx for introducing food into the gut, and undigested food particles are ejected through the mouth. The final stages of digestion are intracellular. Most members of class Turbellaria are predatory on invertebrates smaller than themselves; the rest are herbivores, ectoparasites, or scavengers. Turbellarians use protonephridia scattered throughout their epidermis for the excretion of metabolic waste.

Turbellarians sense their environment with statocysts, chemoreceptors, and photoreceptors. They do not have image-forming eyes, but many species have pigment cells and photoreceptors concentrated into eyespots. The complexity of the turbellarian peripheral nervous system varies from a simple nerve net based on pairs of longitudinal nerve cords to an interlacing web of nerves capable of fine muscular control. Turbellarians are simultaneous hermaphrodites and lay eggs bundled into cocoons. The young undergo direct development and hatch as juveniles. Spiral cleavage is prevalent. Some turbellarians can reproduce asexually by fission. Regeneration of somatic parts is well documented in Dugesia.

Class trematodes

All trematodes are parasitic, and most adult trematodes parasitize vertebrates. Around 9000 species have been described. Their body is covered with a tegument, a peculiar kind of epidermal arrangement in which the main cell bodies are deep, separated from the cytoplasm that lies next to the exterior by a layer of muscle (but connected to the exterior layer by cellular processes. The exterior layer is syncytial; that is, it is continuous, not broken by cell membranes. The tegument lacks cilia in adults. Unlike monogeneans, trematodes have no opisthoaptor; instead, they are characterized by one or two suckers. They are like turbellarians in having a relatively well developed alimentary canal, and their muscular, excretory, and reproductive systems are also relatively complete.

Most trematodes have complex life cycles, with larval stages parasitizing one or more species that are different from host of adults. Larval stages of some medically important species include miracidium, redia, cercaria, and metacercaria. Most trematodes are endoparasites. They include several parasites that have an enormous impact on human populations, such as human liver flukes and the blood flukes that cause schistosomiasis.

Class cestodes

The cestodes, or tapeworms, differ in a number of ways from other flatworms. Their bodies are long and flat, made up of many segments called proglottids. Each proglottid is a reproductive unit, essentially a factory to produce gametes. Adults lack cilia and their surface is a tegument (as in monogeneans and trematodes), but in cestodes the tegument is covered with tiny projections, microvilli, which increase its surface area and thereby its ability to absorb nutrients from a host. Digestive tracts are absent completely. At the tapeworm's anterior end is a specialized segment called a scolex, which is usually covered with hooks or suckers and serves to anchor it to the host.

All of the 5000 or so known species of tapeworms are endoparasites. Most require at least two hosts, with the host of the adult tapeworm a vertebrate. Intermediate hosts are often invertebrates. A number of tapeworm species inhabit humans.

Class monogenea

Monogenetic flukes spend their entire life cycle as parasites on a single host, often on the gills and skin of fish; they include no human parasites. They hold on to the fish by the use of hooks and attachment organs at the posterior end. Most of the parasite's body space is devoted to the hermaphroditic reproductive system. The egg on hatching releases a ciliated larva that enables the parasite to reach a new host. Species of the genus Gyrodactylus can can be serious pests in hatcheries, particularly since a single worm can give rise to more than one hundred descendants in three weeks.