ECHINODERMATA (SPINY-SKINNED INVERTEBRATES)

· Jacob Klein introduced the term echinoderm (echinos = spine; derma = skin) meaning spiny-skinned invertebrates.
· Frey and Leuckart kept these animals in a separate phylum, the Echinodermata.

VVI Examples of Phylum Echinodermata for Entrance Exams are:

Antedon (Sea lily)
Neometra (Feather star)
Asterias (Starfish)
Asterinia
Pentacerous
Ophiothrix (Brittle star)
Ophioderma
Gorgonocephalus (Basket star)
Echinus (Sea–urchin)
Arbacia
Clypeaster (Sand dollar)
Echinocardium (Heart urchin)
Holothuria (Sea-cucumber)
Cucumaria Thyone

Characteristics of Phylum Echinodermata

· Habitat: Exclusively marine, pelagic, or sessile.
· Triploblastic, organ-system level of body organization, deuterostomes, tube within a tube body plan.

A. Body form
· Body is of various shapes like globular, star-shaped, spherical, elongated, and flattened.
· Larva: bilaterally symmetrical,

· Adult: pentamerous and radially symmetrical. (5 antimeres around central axis).

B. Body unsegmented.
· Has distinct oral (mouth) and aboral (anus) surfaces.
· Distinct head is absent.
· Body surface is marked by 5-radiating areas called ambulacral grooves.

C. Body wall
· Outer ciliated epidermis
· Middle dermis consists of a calcareous plate
· Layer of smooth muscles.
· Body bears spines and pedicellariae (for clearing debris) as an exoskeleton.
· Endoskeleton is made up of mesodermal calcareous plates called ossicles present in the dermis.

D. Coelom
· True coelomate; enterocoelomic; surrounded by ciliated peritoneum.
· It develops from archenteron and is present around the alimentary canal and reproductive organs.

E. Water Vascular System (Ambulacral system)
· It is the part of coelom; in the form of radiating canals and tube feet.
· To operate the tube feet, water enters the animal's body through a round hole, the Madreporite.
· Functions of water vascular system: Feeding, locomotion, respiration, excretion.
· Locomotion takes place by Tube feet. It is present on each arm.

Tube feet consist of Ampulla, Podium, and Sucker.

F. Digestive system
· Alimentary canal is complete, which is coiled or straight (exception: incomplete in brittle star).

G. Circulation
· Proper circulatory system absent.
· Open type haemal and perihaemal system.
· Heart absent.
· Respiratory pigments are absent.
· Excretory organ is absent; Ammonotelic.

H. Respiratory system
a) Dermal branchiae or Pappullae in Starfishes.
b) Peristomial gills (in Sea urchins)
c) Genital bursae (in Brittle stars)
· Cloacal respiratory trees are found in Holothurians; the lower branch of which is called Cuverian organs (Respiratory + Protective).
· Tube feet.

I. Nervous system
· Circumoral nerve ring and radial nerves or primitive nerves are present.

J. Reproduction
· Unisexual
· Fertilization is external.
· Development is indirect; larvae are free-swimming ciliated and bilaterally symmetrical.

K. Larval forms

Star fish: Bipinnaria, Branchiolaria
Brittle star: Ophiopluteus
Sea urchin: Pluteus
Sea cucumber: Auricularia
Feather star: Doliolaria
· Asexual reproduction occurs by regeneration.

Classification of Phylum Echinodermata

· Hymen grouped the echinoderms under two subphyla; Pelmatozoa and Eleutherozoa.

A. Subphylum Pelmatozoa is further classified into 5 classes.
Class I : Heterostela (Extinct)

Class II : Cystoidea (Extinct)
Class III : Blastoidea (Extinct)
Class IV : Edrioasteroidea (Extinct)
Class V : Crinoidea (Living)

B. Subphylum Eleutherozoa includes the following classes.
Class I : Asteroidea
Class II : Ophiuroidea
Class III : Echinoidea
Class IV : Holothuroidea
Class V : Ophiocistoidea

ECHINODERMATA

S.N	Pelmaltozoa	Eleutherozoa
1.	These are fixed by a stalk.	These are stalkless.
2.	Their oral surface, directed upwards, both bears mouth and anus.	Anus is present on the aboral surface.
3.	Tube feet ciliated but devoid of suckers.	Tube feet with suckers.

Subphylum: Pelmatozoa

Classes

(a) Crinoidea
(Crion = lily; eiodos = form)
i) Commonly called Sea-lilies or Feather stars.
ii) More or less globular body, fixed to the substratum by central disc or stalked or sessile.
iii) Arms are dichotomously branched and have pinnules.
iv) Mouth and anus are present on the oral surface.
v) Madreporite, spines and pedicillariae are absent.
vi) Ciliated ambulacral grooves on the oral surface.
vii) Tube feet have no suckers.

Examples: Antedon (Sea lily)
Neometra (Feather star)

Subphylum: Eleutherozoa

Classes

(a) Asteroidea
(Aster = star; eidos = form)
i) They are commonly called Starfishes or Sea-stars.
ii) They are benthic, carnivores, and nocturnal. Also, show autotomy.
iii) Body is composed of a flat central disc and radiating arms, not sharply marked off from the central disc.
iv) Pentagonal mouth and ambulacral groove on the oral surface.
v) Madreporite is eccentrically situated on the central disc aborally.
vi) The tube feet and pedicellariae are present.
vii) The endoskeleton is composed of separate ossicles.

Examples: Asterias (Starfish), Asterinia, Pentacerous, etc.

(b) Ophiuroidea
(Ophis = snake; oura = tail; eidos = form)
i) Commonly called brittle stars (autotomy of arms so-called brittle stars).
ii) Five slender arms are sharply marked off from the flat central disc.
iii) The mouth, facing downwards, is situated in the centre of the central disc. The anus is absent.
iv) The madreporite is eccentric and oral.
v) The pedicellariae are absent.
vi) Ambulacral grooves, if present, are covered by sucker-less tube feet.

Examples: Ophiothrix (Brittle star)
Ophioderma
Gorgonocephalus (Basket star)

(c) Echinoidea
(Echinos = hedgehog; eidos = form)
i) The spherical or hemispherical body without arms.
ii) Presence of Jaw suspension called Aristotle's Lantern (Masticatory apparatus/Chewing apparatus).
iii) Ambulacral groove is absent but the sucker-bearing tube feet are present.
iv) Stalked pedicellariae are present.

Examples: Echinus (Sea–urchin), Arbacia, Clypeaster (Sand dollar), Echinocardium (Heart urchin), etc.

(d) Holothuroidea
(Holothurion = sea-cucumber; eidos = form)
i) Elongated, a leathery body without arms, pedicellariae, and spines
ii) Tube feet present but ambulacral groove absent.
iii) Retractile tentacles (modified tube feet) present around the mouth.
iv) Cloaca bears a respiratory tree.
v) Medreporite is internal.

Examples: Holothuria (Sea-cucumber), Cucumaria Thyone, etc.

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