Plathyminthes - Amphibia

• Platyhelminthes


  • Invertebrate organisms, also called flatworms, which have a flat and soft body
  • Majority are parasitic, relying on other organisms for food
  • Some are capable of free living

  
• General characteristics of Platyhelminthes

  • Triploblastic, acoelomate, and bilaterally symmetrical
  • May be free living or parasitic
  • Have a soft covering with or without cilia
  • Body is dorsoventrally flattened without any segments and appears like a leaf
  • Devoid of anus and circulatory system but have a mouth
  • Respire by simple diffusion through the body surface
  • Have an organ level of organization
  • Do not have a digestive tract
  • Are hermaphrodites and can regenerate
  • Reproduce sexually by fusion of gametes and asexually by regeneration through fission

  
• Unique characteristics of Platyhelminthes

  • Parenchyma - a connective tissue which fills in between the body wall and organs, helping in transporting the food material
  • Flame cells - help in excretion and osmoregulation
  • Nervous system comprises the brain and two longitudinal nerve cords arranged in a ladder-like fashion
• Classes of Platyhelminthes

  • Turbellaria - mainly free-living, marine species, although some species live in freshwater or moist terrestrial environments (e.g. Planaria, Otoplana)
  • Trematoda - internal parasites of mollusks and many other groups, including humans (e.g. Fasciola hepatica, Diplozoon)
  • Cestoda - also internal parasites, mainly of vertebrates, that live in the intestinal tract of the primary host (e.g. Taenia spp., Convoluta)

  
• Liver fluke (Fasciola hepatica)


  • Endoparasite that causes a disease called liver rot
  • Found in the bile ducts of sheep, cattle, goats and a few other vertebrates
  • Have no distinguishing behavior traits or social systems, can live solitarily or together in a host

  
• General characteristics of Fasciola hepatica

  • Anterior end is produced into a conical projection called the cephalic cone
  • Mouth is situated ventrally at the anterior end, and it is surrounded by oral sucker
  • Has highly muscular ventral sucker or acetabulum situated a little posterior to the oral sucker
  • Posterior end is large and more rounded in the front than the back
  • Digestive system comprises oesophagus, pharynx and diverticulated intestine
  • Excretory pore is situated at the posterior extremity
  • They are hermaphrodites
  • Eggs are passed to the exterior through the median genital pore present between the ventral and oral suckers
• Life cycle of Fasciola hepatica


  1. Adult fluke lay eggs that are passed out onto pasture in the faeces

  2. At suitable temperatures, a miracidium develops within the egg, hatches and migrates in thin films of moisture, actively seeking the snail host

  3. Miracidia can only survive for a few hours outside the snail

  4. Within the snail they undergo two further developmental stages, including multiplication, eventually becoming infective cercariae, which emerge from the snail when the temperature and moisture levels are suitable

  5. The cercariae migrate onto wet herbage, encysting as metacercariae, the highly resilient infective stage

  6. Following ingestion, the young flukes migrate to the liver, through which they tunnel, causing considerable tissue damage

  7. The infection is patent about 10-12 weeks after the metacercariae are ingested

  8. The whole cycle takes 18-20 weeks

  
• Amphibia


  Any member of the group of vertebrate animals characterized by their ability to exploit both aquatic and terrestrial habitats
• General characteristics of Amphibia

  • Can live both on land and in water
  • Ectothermic animals
  • Body is divided into head and trunk
  • Skin is smooth and rough without any scales, but with glands that make it moist
  • Have no paired fins
  • Have two pairs of limbs for locomotion
  • Three chambered hearts
  • Kidneys are mesonephric
  • Possesses ten pairs of cranial nerves
  • Lateral line is present during their development
  • Respiration is through the lungs and skin, gills might be present externally for adults
  • Sexes are separate and fertilization is usually external
  • Development is indirect with metamorphosis
  • Breeding occurs in water
• Classifications of Amphibians


  • Apoda (Gymnophiona or Caecilia) - "without legs", also known as "blind-worms" because their eyes are covered by skin or bone, tentacles on their head are the chemosensory organs that help them to detect underground prey, possesses venom glands, secretes mucus to reduce water loss
  • Urodela (Caudata) - The body is elongated with four equally sized limbs, the skin is smooth with poison glands, fertilization is internal, feed on insects and worms, found under leaf litter, in the soil, or in water, utilize spermatophores for internal fertilization, possess hidden gills
  • Anura (Salientia or Frogs) - Have four limbs, the front limbs are elongated and modified to jump, the head and trunk are fused together, the tail is present only in the larval stage and is lost in the adults, fertilization is external and the eggs are laid in water

  
• Characteristics of frogs


  • Moist skin for jumping and swimming
  • Long hind legs
  • Lay eggs
  • Ability to live in both land and water
  • Undergo metamorphosis from tadpole to adult frog
  • Some species communicate through vocalizations, such as croaking or chirping
  • Can be nocturnal or diurnal
• Frog breeding


  1. Frogs breed in spring or rainy seasons

  2. Males use loud calls to attract partners by filling vocal sacs with air

  3. When mating, the male frog holds onto the female's back, clasping his front legs around her waist or neck (amplexus)

  4. This ensures the male is in the optimal position to fertilize the female's eggs as she lays them
• There are over 7,000 species of frogs found all over the world with various colors and sizes
• The life cycle of frogs involves metamorphosis from tadpole to adult frog
• Platyhelminthes are traditionally divided into three classes: Turbellaria, Trematoda, and Cestoda
• Fasciola hepatica is an endoparasite that causes a disease called liver rot in sheep, cattle, goats and a few other vertebrates

  
• The whole life cycle of Fasciola hepatica takes 18-20 weeks