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.

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.