Characteristic features of phylum Platyhelminthes

  • Body is dorso-ventrally flattened, leaf like (unsegmented) or tape like (segmented).
  • They are commonly flat worms.
  • They may be free-living like Planarians or parasitic like flukes and tapeworms.
  • They are the first animals to show cephalization and have organ system of body organization.
  1. Body wall:
  • Body is triploblastic and bilaterally symmetrical.
  • Body wall consists of ectoderm, mesoderm and endoderm. Mesoderm splits and forms the parenchymal cells.
  • They are acoelomate animals with no body cavity.
  • The epidermis may be absent and if present, it may be cellular or syncytial or ciliated.
  1. Body systems:
  • The muscular system is well developed which is of mesenchymal origin.
  • It is composed of circular, longitudinal and oblique muscle fibers situated just beneath the epidermis.
  • They have a blind sac body plan with incomplete alimentary canal. Anus is absent and alimentary canal is branched.
  • In tapeworm, digestive system is absent.
  • Skeletal, respiratory and circulatory system are absent. Gaseous exchange occurs through general body surface.
  • Parasitic flatworms possess organs of attachment such as suckers or hooks or both.
  • Excretory system has one or two canals with branches, the finer branches end in flame cells or protonephridia or solenocytes.
  1. Reproduction:
  • They are mostly hermaphrodite (monoecious).
  • Reproductive system is complex with well-developed gonads.
  • In flukes, a pair of testis and a single ovary is present whereas, in tapeworm many testes and a single ovary are present.
  • Asexual reproduction is found in fresh water and land Planarians which takes place by fission.
  • Fertilization is internal.
  • There is cross fertilization in trematodes and self-fertilization in cestodes.
  • Life cycle is complicated with one or more larval stages and one or more hosts.

Phylum Platyhelminthes is divided into three classes as follows/:

Class: Turbellaria

  • They are free-living and ciliated flatworms.
  • Body is covered with epidermis which is provided with mucus cells and rod like bodies called rhabdites.
  • Majority of them are aquatic (marine).
  • They are non-parasitic (independent) forms and the body is unsegmented leaf like.
  • Absence of hooks and suckers.
  • Digestive system is developed with alimentary canal consisting of everted pharynx.
  • They have regeneration capacity.
  • e.g. Planaria (Dugesia), Convulata, Bipalium, Thysanozoon etc.

Class: Trematoda

  • They are commonly called flukes.
  • Body is leaf like and unsegmented, without cilia.
  • They are parasitic (may be ectoparasite or endoparasite).
  • In most cases, life cycle is digenetic (completes in 2 hosts).
  • One or more suckers and hooks are present that help in attachment.
  • Digestive system consists of sectorial mouth, simple pharynx and intestine which is two forked and highly branched.
  • e.g. Fasciola hepatica (liverfluke), Schistosoma mansoni, Polystoma etc.

Class: Cestoda

  • They are endoparasites without cilia and sense organs.
  • They consist of hooks and suckers.
  • Body is segmented (pseudo-segmentation), each segment called a proglottid.
  • Life cycle is digenetic (requiring two hosts to complete the life cycle).
  • Digestive system is absent and predigested nutrients are absorbed from the host’s gut through general body surface.
  • e.g. Taenia solium (tapeworm), Echinococcus granulosus, Hymenolepis nana etc.

Characteristic features of phylum Platyhelminthes