Phylum Echinodermata- The Spiny Skinned Animals

Characteristics of Phylum Echinodermata:

  • Echinoderms are exclusively marine. Most members are bottom dwellers or benthonic. Some are pelagic while a few are sedentary.
  • The adults are all radially symmetrical (near pentamerous) but their larva is bilaterally symmetrical.
  • Echinoderms have organ-system organisation and are triploblastic.
  • Echinoderms are eucoelomates and coelom is enterocoelic (formed as pouches from enteron of gastrula). It is lined with ciliated mesothelium.
  • They possess a water vascular system or ambulacral system which helps in locomotion, capturing and transport of food and respiration.
  • Body is without head or anterior end but is differentiated into oral and aboral surfaces. The oral surface is distinguished into ambulacral and interambulacral regions.
  • Body wall has single layered ciliated epidermis and a thick dermis. Smooth muscles occur below.
  • The digestive system is complete except in class Ophiuroidea. The animals are commonly predaceous and carnivorous but many of them can also feed on organic litter.
  • Respiration occurs by dermal branchiae (gills) or papulae and tube feet.
  • Excretion is partly served by body surface and partly through the dermal branchiae or papulae.
  • Nervous system is simple with a circum-oral ring and radial nerves.
  • Circulatory system is of open type and is called haemal system. Heart is absent. Blood has no pigment.
  • The sexes are separate. Fertilization is usually external. Development is indirect.
  • They possess remarkable power of autonomy and regeneration.
Key Point- The term echinoderm was coined by Jacob Klein in 1734.

Unique Features of Phylum Echinodermata:

  • Occurrence of ambulacral or water vascular system.
  • Presence of haemal system.
  • True enterocoelom.
  • Presnce of spines and pedicellariae.
  • Presence of tube feet.
  • Mesodermal endoskeleton.
  • Change in symmetry from bilateral in larva to pentamerous radial in adult.
  • Ability of self shedding of body parts or autonomy and regeneration.
  • Deuterostomic origin of mouth.

Degenerate Features of Phylum Echinodermata:

  • Absence of head.
  • Reduced circulatory system.
  • Incomplete digestive tract in Ophiuroidea.
  • Absence of respiratory pigment.
  • Absence of excretory organs.
  • Poorly developed nervous system.
  • Simplification of sense organ.

The degenerated features are mainly associated with their existence on sea bottom.

Resemblance with Chordates:

Echinoderms, although, so different but are nearest relatives of chordates. Their chordate characters are-

  • Both echinoderms and chordates are deuterostomes.
  • Have enterocoelic coelom.
  • Both have indeterminate cleavage and the cleavage furrows are radial.
  • Endoskeleton is mesodermal.
  • Mouth is formed as a second opening by the invagination of ectoderm at the anterior end in the embryo, opposite to blastopore.
  • Occurrence of creatine phosphate as an energy source for muscle activity.

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