Phylum Echinodermata

Phylum Echinodermata

- Bilateral and deuterostomial eucoelomate eumetazoa with rough, tough and leathery skin due to dermal skeleton of usually spiny, calcareous ossicles; and the basic bilateral symmetry of larva modified, in the adult, to a pentamerous radial symmetry.

- The term “Echinodermata” means spiny skin (Gr., echinos = spiny + dermatos = skin).

- About 6,000 living and 20,00 extinct species known.
 

Brief History

- Although Jacob Klein (1738) had earlier coined the name “Echinodermata”, yet Linnaeus included these animals under “Mollusca”, and Lamarck under his class “Radiata” as “Echinodermes”.

- Finally, Leuckart (1847) raised the group to the status of a separate phylum.

Salient features

(1) Echinoderms are exclusively marine beings.

(2) They are triplobalstic and coelomate animals.

(3) They have radially symmetrical body. The radial symmetry is due to sedentary or sessile mode of life and it is a secondary character in echinoderms.



(4) They have organ system grade of organization.

(5) They have well developed endoskeleton formed of calcareous ossicles and spines.

(6) They have a water–vascular system with tube–feet for locomotion, feeding and respiration.

(7) Circulatory system is of the open–type.

(8) The sensory organs are poorly developed.

(9) The excretory organs are absent.

(10) They have pedicellariae.

(11) Development is indirect.

(12) The larval forms are bilaterally symmetrical.

(13) Regeneration power is well developed in Echinoderms.

(14) Water vascular system found in echinodermata
 

Classification of Echinodermata

Subphylum I. Eleutherozoa ­: Free-living echinoderms.

Class 1. Asteroidea

(1) Starfishes or sea stars.

(2) Arms 5 or more and not sharply marked off from the central disc.

(3) Tube feet in orally placed ambulacral grooves; with suckers.

(4) Anus and madreporite aboral.

(5) Pedicellariae present.

(6) Free-living, slow-creeping, predaceous and scavengerous.

(7) Examples : Astropecten, Luidia, Goniaster, Oreaster (= Pentaceros), Asterina, Solaster, Pteraster, Echinaster.

Class 2. Ophiuroidea

(1) Brittle-stars and allies.

(2) Body star-like with arms sharply marked off from the central disc.

(3) Pedicellariae absent.

(4) Stomach sac-like; no anus.

(5) Ambulacral grooves absent or covered by ossicles; tube feet without suckers.

(6) Madreporite oral.

(7) Examples : Ophiura, Ophiothrix, Ophioderma, Ophiopholis, Gorgonocephalus (basket star), Asteronyx.

Class 3. Echinoidea

(1) Body not divided into arms; globular (sea urchins), or flattened disc-like (sea-cakes).

(2) Mouth at lower pole, covered by 5 strong and sharp teeth, forming a biting and chewing apparatus called “Aristotle's Lantern”.

(3) Tube-feet slender with suckers.

(4) Skin ossicles fused to form a rigid globular, disc like, or heart-shaped shell or test with movable spines.

(5) 3–jawed pedicellariae present in skin.

(6) Gut long, slender and coiled. Anus present.

(7) Larval forms pluteus and Echinopluteus.

(8) Examples : Sea urchins and sand dollars.etc.

Class 4. Holothuroidea

(1) Body massive, long and cylindrical like a cucumber; elongated in oral–aboral axis; no arms.

(2) Mouth at anterior and anus at posterior ends.

(3) Mouth surrounded by many hollow retractile tentacles.

(4) Tube feet usually present; sucker-like.

(5) Skin leathery, but relatively soft, without spines or pedicellariae; may have an endoskeleton of miniute calcareous ossicles.

(6) Respiration and excretion by two long and highly branched tubes (= respiratory tree) extending into coelom from cloaca.

(7) Larval form Auricularia.

(8) Examples : Holothuria, Cucumaria etc.

Subphylum II. Pelmatozoa

Stalked, sedentary echinoderms.

Class 5. Crinoidea

(1) Body flattened and pentamerous; distinguished into a small and circular central disc and five or more (in multiples of five) long, then, branched and flexible arms radiating from the disc.

(2) Disc enclosed in a hard, cup–shaped calyx formed of calcareous plates; calyx attached to a substratum by a stalk or simply by its aboral surface.

(3) Mouth in middle and anus excentral upon a cone, both upon oral surface. 5 ambulacral grooves run from mouth upto the tips of the arms.

(4) Tube feet sucker–like; restricted to central disc; can help in food–collection.

(5) Some forms (sea–lilies) permanently sessile and attached to sea–bottom by a long stalk; others (feather stars) free-swimming, but have flexible cirri for gripping objects in water.

(6) Spines and pedicellariae absent in skin.

(7) Examples : Sea lilies and Feather stars (Antedon)