Phylum Echinodermata, Biology tutorial


An extraordinary feature of group is pentamerous radial symmetry. This means that body can be separated into 5 equal parts arranged around central axis. But die radial symmetry of adult echinodemts is secondary. Larvae of eclimoderms are bilaterally symmetrical. In metamorphosis bilateral symmetry of larvae is replaced by radial symmetry in adults. Echinoderms do not have relationship with other radially symmetrical invertebrates. Another feature of echinoderms is presence of internal skeleton formed of calcareous ossicles. Ossicles may articulate with one another or they may be sutured together. Spines of tubercles project out front the surface of the body that gives the animal a warty or spiny appearance and therefore name of phylum Echinodernmata meaning spiny skinned animals.

Echinoderms can lose their body parts by autonomy and have power of regeneration also. Phylum Echinodemmata comprises five classes. They are:

i) Class Asteroidea

ii) Class Ophiuroidea

iii) Class Echinoidea

iv) Class Holothuroidea

v) Class Crinidea

The Features of Phylum Echinodermata are:

1. Echinodermata is deuterostomous phylum. Adults generally with pentamerous symmetry; all are marine.

2. Body not metamerically segmented; rounded, cylindrical or star shaped, without head.

3. No brain. Few specialized sense organs. Central nervous system in form of circumneutral ring and radial nerves.

4. Coelom enterocoelic and extensive. Forms perivisceral cavity and cavity of unique water vascular system.

5. Locomotion mostly by tube feet; in some by spines or by movement of arms.

6. Respiration by dermal branchiae, tube feet, respiratory tree or bursae.

7. do not have olfactory organs.

8. Sexes generally separate; gonads large simple gonoducts. Fertilization external.

Class Asteroidea:

Star shaped; arms not sharply demarcated from central disc; ambulacral grooves open; tube feet on oral side and with suckers. Arms and madreporife on aboral side; pediceltaria are present. Class comprises sea stars or starfishes. Sea stars are variously colored free living animals common along rocky shores.

Body has the central disc from which 5 arms project out giving animal pentamerous radial symmetry. They range from 12 to 24 cm on an average. Arms are not very diverse from central disc; rather they imperceptibly grade in central disc. Mouth is located at centre of disc on its under surface that is therefore known as oral surface. From the mouth a furrow known as ambulacral groove extends in each arm radially.

The groove harbours two or four rows of small tubular projections known as tube feet or podia. Groove may be kept partially closed by movable spines which are located on margins. Each arm bears at its tip one or more tentatcle like sensory tube feet and red pigment spot. Upper surface of animal is known as aboral surface and it bears anus at centre. Ossicles are composed of magnesium rich calcite. Dermis is followed by muscles which help in bending of arms. The coelom is lined by the ciliated peritoneal membrane.

Water Vascular System:

Echinoderms have unique system which comprises of canals and appendages of body wall called water vascular system. This system is coelomic derivative: it is lined with ciliated epithelium and filled with fluid. In asteroids water vascular system is well developed and is mostly locomotor and food gathering in function. It also acts in respiration and excretion. Water vascular system communicates with exterior by button shaped madreporite situated on aboral surface. Madreporite has a number of pores on it.

Digestive System:

Most asteroids are scavengers or carnivores and feed on snails, bivalves, polychaets, and other echinoderms, fish, sponges, sea anemones and polyps of hydroids and corals. Few feed on plankton and detritus. Digestive system of sea stars extends between oral end and aboral disc of animal. Mouth lies at centre of muscular peristomial membrane.

Mouth is followed by the short oesophagus that in turn opens in large stomach. Stomach occupies most of the interior of disc and is separated by horizontal constriction into large oval cardiac stomach and smaller, flattened pyloric stomach. Walls of gastric stomach are pouched. In each arm a pair of digestive glands or pyloric caeca is present and their ducts open into pyloric stomach.

Circulation, Respiration and Excretion:

Asteroids depend on circulation of coeloimic fluid for transport of gases and some nutrients. Blood vascular system known as haemal system in echinoderms is not very well developed in asteroids. Channels lack a lining. Though, these channels are enclosed by extensions of coelom known as perihaemal sinuses. There are four coelomic circulatory systems in asteroids

1) The perivisceral coelom and fluid which circulates in it. Supplies the viscera

2) Water vascular system which supplies muscles of tube feet that helps in locomotion

3) Hyponeural sinus system which supplies nervous system

4) Genital coelom which supplies genital organs.

Nervous System:

Nervous system of steroids is not ganglionated and is closely related with epidermis. It comprises mostly of circumoral nerve ring surrounding mouth. From this nerve ring radial nerves extend in each arm. Radial nerve supplies branches to podia and ampullae and is continuous with subepidermal nerve net. Sense organs in asteroids are eye spots found at tip of arm. Epidermis has also sensory cells which maybe function as photoreceptors, chemoreceptors and mechanoreceptors. Epidermal sensory cells are more prevalent on suckers of tube feet.

Reproductive System:

Sexes are separate in most asteroids. There are ten gonads, two in each arm. They appear tuft-like or as cluster of grapes. Mature gonads occupy almost whole arm. Gonopore is situated between bases of die arms. In majority of seastars sperms and eggs are shed in sea water and fertilization is external. Single female may shed as many as 2.5 million eggs. Development comprises larval stage - the bipinnaria lava

Class Ophiuroidea:

Body star shaped but arms sharply separated from central disc; ambulacral grooves enclosed by ossicles; tube feet do not have suckers; pedicellaria and arms. It comprises brittle stars, the largest class of echinoderms in terms of number of species. They inhabit benthic zones of sea. Tube feet lack suckers, and therefore they are useful in feeding but have no role in locomotion. Unlike in asteroids, in ophiuroids madreporite is located on one of the oral shield ossicles.

Integument of animal is leathery and harbours dermal plates and spines. Visceral mass is confined to central disc. There is sac-like stomach and no intestine. Anus is absent. Undigested food is discarded by mouth. Nervous and haemral systems are like those of asteroids. Central disc harbors five pairs of sac like structures known as bursae found only in ophiuroids among echinoderms.

Class Echinoidea:

Globular or disc shaped, devoid of arms. Skeleton compact composed of dermal ossicles closely fitted with one another. Spines movable; ambulacral grooves closed; tube feet having suckers; pedicellariae present.

Class Echinoidea comprises sea urchins sand dollars and heart urchins. Echinoids have compact body that is surrounded in endoskeletal test or shell. Test is composed of dermal ossicles that are closely sutured with each other forming compact structure. Plates bear stiff movable spines. Five pairs of ambulacral rows contain pores and are homologous to five arms of star fishes. Through pores long tube feet are protruded. Echinoids also have pedicellariae that aid animals to keep body clean and to capture small organisms. Mouth of sea urchins lies at centre of 5 converging teeth. Teeth are part of complex chewing organ known as Aristotle's lantern. Sea urchins feed algae and other organic materials.

Class Holothuroidea:

Body is like cucumber; does not have arms, spines and pedicellariae; ossciles minute and embedded in muscular wall; anus present; tube feet with suckers; ambulacral grooves closed; altered tube feet in form of circumoral tentacles present; madreporite internal. Holothuroida are usually called as sea cucumbers due to their resemblence to cucumber. They vary from rest of echinoderms in several respects. Holothurians have really lengthened oralaboral axis.

In some species however tube feet may be distributed in 5 ambulacral areas or all over the body. There are between 10 to 30 retractile oral tentacles which are modified tube feet. Sea cucumbers are slow animals. Suspended food particles are entangled in mucus secreted by oral tentacles. Tentacles are, then stuffed on by one through mouth in pharynx and food particles are sucked off.

Class Crinoidea:

Five arms but branching at base; they carry pinnules. Oral surface contains ciliated ambulacral grooves with tube feet resembling tentacles. Does not have spines, madreporite, pedicellaria. Class Crinoidea comprises sea lilies and feather stars. They comprise the most primitive of living echinoderms. More fossil forms are known than living ones. Also feather stars that remain sesile or stalked during metamorphosis, detach, themselves from stalk to become free living. Several crinoids live at great depth but few of the feather stars occupy shallow waters. Body disc of crinoids known as calyx is enclosed by leathery skin, tegmen. Tegmen has calcareous plates. There are 5 flexible arms that branch off to give rise to more arms. Each arm carries several lateral pinnules which are arranged like barbs on deather. Calyx and arms together are known as crown. In forms that are attached, there is long stalk present on aboral side of body. Stalk itself is formed of several jointed plates and may bear cirri. In crinoids madreporite. Spines and pedicellariae are not present.

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