Platyhelminthes : General Characters Classification Upto Classes

 Introduction

Platyhelminthes are triploblastic, bilaterally symmetrical, dorsoventrally flattened, acoelomate flatworms with organ grade of construction without a definite anus, circulatory, skeletal or respiratory system but with Protonephridial excretory system and mesenchyme filling the space between the various organ of the body.

Phylum Platyhelminthes (flatworms) characteristics :

• They are free-living, commensal or parasitic.
• They are bilaterally symmetrical and dorsoventrally flattened, triploblastic worm.
• Bilaterally symmetrical with the definite polarity of head and tail ends.
• Triploblastic i.e. body derived from three embryonic germ layers; ectoderm, mesoderm, and endoderm.
• Dorsoventrally fattened i.e. well-developed ventral surface with mouth and gonopore. 
•  Their body generally shapes as a worm but varies from moderately elongated flattened to long ribbon-like and leaf-like.
• They are small to moderate in size varying from microscopic to extremely elongated form measuring up to 10-15 meters.
• Their body is unsegmented except in class Cestoda.
• The majority of them are white, colorless and some derive color from ingested food while free-living form are grey, brown-black or brilliantly colored.
• Their anterior end of the body is differentiated into the head.
• Mouth and genital pores on the ventral surface are well marked in turbellarians but less marked in cestodes and trematodes.
• Their parasitic form has adhesive structures like hooks, spines and suckers, and adhesive secretions.
• The body is covered with cellular or syncytial, frequently ciliated epidermis; while trematodes cestodes, lacks epidermis and their body covered with cuticle.
• Exo- and endoskeleton are completely absent, hence the body is generally soft. The hard part consists of cuticle, spines, thorns, hooks, teeth.
• They are acoelomate i.e. without any body cavity.
• Space between various organs filled with special mesodermal tissues, the mesenchyme, and parenchyma.
• Their digestive system is branched and incomplete without an anus and totally absent in acoela and cestode.
• They lack skeletal, respiratory and circulatory systems.
• Their sense organs are simple. A common occurrence in tubellaria but greatly reduced in parasitic form. Chemo- and tangoreceptors commonly in the form of ciliated pits and grooves.
• They are mostly monoecious (hermaphrodite).
• Their reproductive system is highly evolved or complex in most of the forms.
• Asexual reproduction occurs by fission in many freshwater turbellaria.
• In the majority of form, eggs are devoid of yolk. They are produced separately in the yolk or vitelline glands. The excretory system includes a lateral canal and a single or pair of protonephridia with flame cells or bulbs. Absent in some primitive form.
• Their nervous system is primitive, ladder-like. The main nervous system consists of a pair of ganglia or brain and one or three pairs of longitudinal nerve cords connected by transverse nerves. 
• Fertilization is internal but cross-fertilization in trematodes and self-fertilization in cestodes.
• Their life cycle is complicated involves one or more hosts.
• Parthenogenesis and polyembryony commonly occur trematodes and tapeworms.
• Some tapeworm propagates by endogenous or exogenous budding.
• The flatworm is either free-living or ecto-or endocommensals or parasitic. 

Phylum Platyhelminthes (flatworms) Classification:

The classification is from Hyman, L.H., (1951) up to suborder only with certain modifications.

Class 1- Turbellaria (L., turbella= a little string) 

• Mostly free-living but some ectocommensals and endocommensals or parasitic called.
• Terrestrial marine or freshwater.
• Body unsegmented and covered with ciliated cellular or syncytial epidermis, containing mucus-secreting cells and rod-shaped body called.
• Mouth ventral. intestine preceded by the muscular pharynx.
• Adhesive organs(suckers) abundantly present.
• Sense organ i.e. Tango, chemo, and photoreceptors common in free living forms.
• The excretory system consists of protonephridia, the flame cells.
• Mostly reproduction sexual, asexual and by regeneration.
• Life cycle simple. 

Class 2- Trematoda (Gr., trematodes= having pore) 

• Ectoparasitic or endoparasitic commonly called.
• Body unsegmented dorsoventrally flattened leaf-like.
• Teguments thick but without cilia and rhabdites.
• Body undivided and covered with cuticle.
• Suckers and sometimes hooks present.
• Digestive tract incomplete consists of the anterior mouth, simple pharynx and two forked or many branches intestine; anus absent.
• 3 pairs of the longitudinal nerve cord.
• Protonephridial excretory system consisting of flame cells.
• Mostly hermaphrodites(monoecious).
  Single ovary, 2 to many testes.
• Development direct (in ectoparasites) or indirect (in endoparasites) with alternation of hosts.

Class 3- Cestoda (Gr., ketos, gridle+ eidos, form) 

• Endoparasitic in the intestine of vertebrates.
• Commonly called tapeworm.
• Body divided into many segmented (proglottids) but rarely undivided, elongated, flat, ribbon-like.
• Tegument without microvilli.
• Body without epidermis and cilia but covered with cuticle.
• Anterior end (scolex) is provided with adhesive structures (hooks, suckers) except in cestodaria. • Mouth and digestive systems totally absent.
• The excretory system consists of protonephridia with typical terminal flame
• The nervous system usually comprises a pair of ganglia and 2 lateral longitudinal nerve cords.
• Each mature segment or proglottids monoecious, with male and female organs.
• Life cycle complicates usually involving 2 or more hosts.
• Embryos with hooks.