PHYLUM-ECHINODERMATA
Phvlum—Echinodermata (Echinos —spines; derma—skin)
includes the spiny skinned animals, which are exclusively
marine. Jacob Klein ( 1734) coined the term ‘Echinodermata’.
It includes about species.
General Features
Some important general features of phylum—Echinodermata are
discussed below
(i) Habit and Habitat These are marine forms and are
bottom dwellers.
(ii) Symmetry The adults have radial (pentamerous)
symmetry but, the larval forms have bilateral symmetry.
(iii) Germ Layers and Organisation They are triploblastic
and exhibit organ system grade of organisation.
iv) Head It is absent in echinoderm and body also lacks
segmentation.
(v) Body Cavity They have true coelom, which is
lined by a ciliated peritoneum. The most
distinctive feature is the presence of water
vascular system or ambulacral system with tube
feet that help in locomotion, capture of food and
respiration.
(vi) Endoskeleton It contains numerous calcareous
plates called ossicles beneath the skin.
(vii) Locomotion The locomotion is carried out by
tube feet.
(viii) Digestion The digestive tract is simple and
complete type. The mouth is present on the
lower side and the anus is on the upper side.
(ix) Respiration It occurs through tube feet, which
help in respiration.
(x) Circulation It is reduced and open type called as
haemal system.
(xi) Excretion The excretory organs are absent. The
waste products are removed by diffusion through
branchiae or amoebocytes.
(xii) Nervous System It contains circum-oral nerve
ring, transverse and radial nerves.
xiii. Reproduction Echinoderms reproduce sexually
Sexes are separate and show no sexual
dimorphism. Fertilisation is external and
development is indirect, e.g., Asterias, (seastar or
starfish), Ophiura (brittle star), Echinu
(sea urchin),
Cucumaria (sea cucumber), Antedon (feather star
or sea lily).