Introduction to Phylum Echinodermata:
Phylum Echinodermata group comprises sea urchins, star fishes, sea-lilies and others, all of which dwell in sea. Some Echinoderms crawl slowly in shallow waters along shore or lie partly concealed in holes of rooks while other like sea-lilies are located attached deep down at bottom. Phylum comprises some 6000 species. Echinoderms have the vascular system which holds water and which in few echinoderms end in suckered feet allowing creature to hold and move objects. Reproduction in Echinoderms reproduction is generally by means of external fertilization through eggs and sperm discharged into water, and majority of echinoderms have many planktonic larval stages prior to reverting to the sessile existence on seafloor.
As most echinoderms contain some kind of calcareous support exoskeleton (frequently interlocking plates of calcium carbonate), there exists the extensive fossil record tracing echinoderm evolution. Still, several aspects of the early evolutionary origins are confounded.
Features of Phylum Echinodermata:
i) All Echinoderms are marine animals, and adults are mostly with Pentamerous radial symmetry (i.e. can be separated into five equal parts). Body isn't metamerically segmented;
ii) They do not have brain, only some specialized sense organs.
iii) They have complete digestive system.
iv) Rounded, cylindrical or star shaped without head.
v) Locomotive is primarily by tube feet, in some by spines or by movement of arms.
vi) They do not have olfactory organs.
vii) Sexes are separate, fertilization is external.
viii) They have indeterminate kind of development.
ix) Respiration is by dermal branchiae, tube feet or respiratory trees.
x) They have the internal skeleton (endoskeleton) composed of plates of calcium carbonate, imbedded in the body wall.
Categorization of Echinoderms:
Phylum Echinodermata comprises of five classes given below:
i) Class Asteroidea e.g. Star Fishes
ii) Class Opliuroidea e.g. Brittle Stars
iii) Class Echinoidea e.g. Sea Urchiris
iv) Class Holothuroidea e.g. Sea-Cucumbers
v) Class Crinoides e.g. Sea Lilies
External Features of Starfish:
i) They are star shaped.
ii) Arms aren't sharply demarcated from central disc;
iii) Tube feet are on oral side and with suckers
iv) Arms and Madreporite (the hole by which water enters body) are on aboral side (upper surface of animal)
v) Pedicellaria are there (this are specialized jaw-like or pincer like appendages).
Ecological Adaptation of Starfish:
i) Possession of specialized jaw-like or pincer like appendages on the body surface for protection of animals from small animals and larvae which try to settle on sea stars.
ii) Possession of the well developed water vascular system that functions as locomotors and for food gathering.
iii) Water vascular system provides respiration and excretion.
iv) Many are scavengers or carnivores feedings on snails, other echinoderms, sponges, fish, sea anemones etc. few feeds on plankton.
v) Possession of Tube feet that also, mainly serves as respiratory organs.
vi) One of the strange adaptations of starfish is their skill to evert the stomachs. When the starfish captures the clam or similar bivalve, it holds it with its arms and pries it open a little. Then, starfish everts the stomach, sliding it in shell of prey. Stomach then digests prey and retracts back in original form.
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