BY JAMES BURROWS Platyhelminthes

Whats a Platyhelminthes? Platyhelminthes is a phyla or category of a species of organisms. These organisms are known for their worm-like structure. However they contain complex organ systems. Its origin came from Platy (meaning flat) and helminthes (worm) in Latin.

Characteristics of this Phyla are: Characteristics of Platyhelminthes:- 1)Bilaterally symmetrical. 2)Body having 3 layers of tissues with organs and organelles. 3)Body contains no internal cavity. 4)Possesses a blind gut 5)Has Protonephridial excretory organs instead of an anus. 6)Has normally a nervous system of longitudinal fibres rather than a net. 7)Generally dorsoventrally flattened. 8)Reproduction mostly sexual as hermaphrodites. 9)Mostly they feed on animals and other smaller life forms. 10)Some species occur in all major habitats, including many as parasites of other animals.

Species of Flatworms (Platyhelminthes) The Platyhelminthes are a successful phylum with around 25,000 known species divided into four classes. Most Platyhelminthes are parasites on other animals, only the Turbellarians are mostly non- parasitic. A few species are commensalists living in harmony, or mutual benefit with another, normally larger organism. Most species feed on animal material either as parasites or as scavengers, a very few species feed on algae. Although a few of the free living marine and terrestrial species are very beautiful, most species are not particularly attractive to the human mind.

Fossil Records Platyhelminths have practically no fossil record. A few trace fossils have been reported that were probably made by platyhelminths (Alessandrello et al., 1988), and fossil trematode eggs have been found in Egyptian mummies and in the dried dung of Pleistocene ground sloth.

Life Cycle Most have very complex lifecycles with up to seven stages, depending on what combinations of environments the early stages encounter – most importantly whether the eggs are deposited on land or in water. The intermediate stages transfer the parasites from one host to another. The definitive host in which adults develop is a land vertebrate, the earliest host of juvenile stages is usually a snail that may live on land or in water, and in many cases a fish or arthropod is the second host.

Life Expectancy There life cycle is of about 3 to 4 months in optimum and suitable conditions. but when condition around them is not proper for there next generation or growth they undergo a process wherein they form tough cover around them and remain as it is until and unless they got proper conditions to grow and this can be too long even 100 to 200 years.

Curiosities: Liver Fluke Liver flukes are a polyphyletic group of trematodes (phylum Platyhelminthes). Adults of liver flukes are localized in the liver of various mammals, including humans. These flatworms can occur in bile ducts, gallbladder, and liver parenchyma. They feed on blood. Adult flukes produce eggs which are passed into the intestine.

Digestive and Reproductive Systems Flatworms have three distinct layers of tissue, all composed of living cells. They have true organs and organ systems for digestion, movement, excretion, and reproduction. The digestive system consists of a muscular tube with one opening at the mouth. The excretory system consists of a network of water- collecting tubules that empty their contents into sacs leading to the exterior.

Sensory System The “head” contains a mass of nerve cells that acts as a brain and specialized regions for sensing light, chemicals, and pressure. 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 are no special organs for respiration and circulation.

Curiosity: Asexual and Sexual? Both trematodes and cestodes exhibit alternation of generations, that switching back-and-forth between sexual and asexual reproduction. As parasites in all stages, the asexual stages take place inintermediate hosts and the sexual stages take place in final or definitive hosts. One would think since this approach produces both of the reproductive types' advantages (high variation plus ability to copy successful forms), it would be fairly common, but in practice it becomes an extremely complicated life cycle.