⚫ “Platyhelminthes” means flatworms (Gr., platys = flat; helmins = worms); their body is dorsoventrally flattened.
⚫ Salient features:
- They are dorsoventrally flattened like a leaf
- They show organ grade of organization
- They are acoelomate animals
- They are triploblastic animals.
- They are bilaterally symmetrical animals.
- Some members have segmented body.
- Many of the parenchyma cells give rise to muscle fibres.
- The digestive system is completely absent from Cestoda and Acoela.
- The respiratory organs are absent. In parasites respiration is anaerobic
- There is no circulatory system
- The excretory system is formed of protonephridia (flame cells)
- The nervous system is well developed.
- They are hermaphrodites, i.e., both male and female reproductive organs are present in the same animal
- Fertilization is internal in them.
- They are free-living or parasitic.
⚫ Classification of Platyhelminthes
Class 1 – Turbellaria
- Most of the turbellarians are free living but some of them are ecotocommensal or parasitic
- The body epidermis is either cellular or syncytial
- Segmentation is absent
- Digestive system is present except in a few
- Suckers are absent
- Life cycle is simple
- Example: Dugesia, Notoplana, Bipalium
Class (2) – Trematoda
- Ecto or endoparasites of vertebrates; commonly called flukes.
- Body mostly oval, unsegmented.
- Body wall without cilia, but covered by a thick, resistant, syncytial tegument.
- Suckers, and often hooks and spines, present for attachment to host tissues.
- Sense organs usually absent in adults.
- Digestive system well developed with terminal mouth, but no anus.
- Mostly hermaphrodite. Life cycle is simple or complicated.
- Examples: Polystomum, Fasciola, Schistosoma (blood fluke of man and other mammals).
Class (3) – Cestoda
- All are endoparasites. Mostly found in alimentary canal of vertebrates; commonly called tapeworms.
- Body long and slender, tape-like, usually divided into small segments
- Body wall non-ciliated, with a thick tegument.
- Anterior end with suckers and other attachment organs.
- No mouth; digestive system absent.
- Sense organs absent.
- Each proglottid contains one or two complete sets of hermaphrodite (bisexual) reproductive organs.
- Life-cycle usually complicated with alternation of hosts. Embryo hooked.
- Examples – Taenia, Echinococcus, Hymenolepis.
Phylum Nematoda (or Nemanthelminthes)
⚫ The term “Nematoda” literally means “threadworms” or “roundworms” (Gr., nema = thread + eidos = form).
⚫ Salient features
- Many endoparasites of various animals and plants.
- Mostly minute or small; some large (1 mm to 25 cm); some upto several metres long.
- Slender, cylindrical, elongated body usually tapering towards both ends, and non-segmented.
- Body wall formed of a thick, tough and shiny cuticle.
- The false body cavity, or pseudocoel is spacious, with a fluid but no free cells
- Straight alimentary tract with terminal mouth and anus
- Circulatory system and respiratory organs absent.
- They are usually unisexual with sexual dimorphism.
⚫ Classification of Nematoda: On basis of the presence of absence of some specialized sense organs and caudal glands, and characteristics of excretory system, nematodes are classified into two classes:
Class (1) – Phasmidia or Secernentea:
- Mostly parasitic.
- Possess a pair of unicellular, pouch-like sense organs, called plasmids.
- Another pair of reduced, pore–like sense organs, called amphids, present near anterior end.
- Excretory system with paired lateral canals.
- Caudal glands absent.
- Examples – Ascaris, Enterobius, Ancylostoma, Wuchereria, etc.
Class (2) – Aphasmidia or Adenophorea:
- Mostly small, free-living.
- No phasmids.
- Amphids spiral, cord like or disc like, seldom pore like.
- No lateral excretory canals.
- Caudal glands present.
- (6) Examples – Tichinella, Capillaria etc.