Unit 3: Platyhelminthes

Unit 3: Platyhelminthes General characteristics and classification up to classes. Life cycle and pathogenicity of Fasciola hepatica and Taenia solium. Unit 3 Q. General characters of Platyhelminthes: The word platyhelminthes means "platy" (flat) and "helminthes" means worms. The organism may be free-swimming or parasitic. Body dorsoventrally flattened. Bilaterally symmetrical body. Morphology may be tape-like or leaf-like. Triploblastic body (ectoderm, mesoderm, endoderm) Spines and hooks may be present. Body covered with cuticle (made up of cellulose, protein, and calcium) Epidermis may contain sucker (nipple-like) Alimentary canal may be absent or highly branched Respiratory system, circulatory system absent. Nervous system may be poorly developed. Maximum organisms are hermaphrodite. Fertilisation internal Development may be direct or indirect Alternation of generation may be seen. Example: Fasciola, Taenia 2) Classification of Platyhelminthes up to classes: The organisms of this group are classified as follows: Class-I Turbellaria They are mostly free-living worms, fresh water or sea water, on land. Body dorsoventrally flattened Hook-like suckers are absent Development is indirect Highly branched alimentary canal Example: Haplodiscus, Convoluta Class-2 Trematoda All are parasites (ectoparasites, endoparasites) Unsegmented body. Body covered with thick cuticle. Cilia, hooks are absent Adhesive organs are the suckers. Digestive system with intestine, anus absent. Life cycle is simple with one or more hosts Hermaphrodites Example: Fasciola, Polystomion Class-3 Cestoda The organisms are totally endoparasitic. Body is elongated and flattened Cestoda: The organisms are ribbon-like Body is covered by thick cuticle, but cilia are absent Body has an important part called scolex Mouth, digestive tract are absent Sexes are united (hermaphroditic) Self and cross-fertilization is common Example: Amphilinia, Taenia, Aporina Life cycle and pathogenicity in Fasciola hepatica: Fasciola is commonly known as liver fluke It comes under the class trematoda The organism is leaf-like The common species is Fasciola hepatica It is distributed all over the world It is commonly also called digenetic parasite Life cycle: Fasciola hepatica is hermaphrodite It is a digenetic parasite The primary host is mammal, and the secondary host is freshwater snail (Pila, Lymnaea) It is an endoparasite, hence completes its life cycle in two hosts The Fasciola hepatica attaches itself to the wall of the bile duct with the help of suckers. The adult fluke produces a large number of eggs. Though Fasciola is hermaphrodite, cross-fertilization occurs. Before fertilization, the first step is copulation; copulation occurs in the bile duct of sheep. Then the sperm from the male deposits in the oviduct of the female. Formation of Egg Capsule: The eggs are brownish in color, oval-shaped. Eggs are fertilized in the oviduct. The eggs are enclosed in a shell, which is also called a capsule. In general, also, the egg shell is called a capsule. The capsule reaches the intestine from the bile duct. From the intestine, the capsules come out with feces (waste material). The capsules finally fall at any place. Development of Fasciola hepatica The fertilized eggs now start developing through a number of larval stages. 1. Miracidium Larva: It is a minute larva. It is elongated in shape. It is a free-swimming stage. It is covered by 18-plate ciliated epidermal cells. The miracidium larva undergoes various changes where it makes a suitable environment. After getting a suitable environment, it can grow further. The miracidium larva stops feeding unless it reaches an intermediate host. The intermediate host is a snail (Linnaea) (Name of the freshwater snail). The miracidium enters into the pulmonary sac of the snail. It can finally reach the digestive gland of the snail. Then the covering is broken down and it further develops into a sporocyst larva. 2. Sporocyst Larva: It is also an elongated shape. The sporocyst has protonephridia germ cells. and germ balls. The sporocyst moves to the tissue of the host and the germ cells develop into another larva, called the Redia larva. 3. Redia Larva: A sporocyst can produce 5-8 Redia larvae. These are also elongated in structure. The Redia larvae come out from the sporocyst and now they migrate to the liver of the snail. These larvae develop mouth, pharynx, intestine, and protonephridia with flame cells (flame cells are also the unit of excretory system). The germ cells can further develop in another stage of the larva which are called Cercaria larvae. 4. Cercaria Larva: It is an oval body. It forms a covering around it called cuticle. Oral sucker starts developing. Excretory canal fully developed. Excretory pores fully developed.