Question

Describe the characteristics of Phylum Echinodermata and discuss how the body form varies in the 5 classes

zoology question

Answer 1

Habitat: Echinoderms are marine. They grow at sea bottom borne are pelagic (free swimming in open water) and a few are sessile (attached to the substratum).

Body Form: It varies considerably. The body is star-shaped, spherical or cylindri­cal. It is un-segmented. The body do not have head.

Spines and Pedicellariae: Many echinoderms has spines and pincer-like pedicellariae on their skin. The spines are protective in function. The pedicellariae keep the body surface clear of debris and minute organisms.

Symmetry: The symmetry is bilateral in larvae but pentamerous radial in adults i. e., body parts are arranged in fives or multiples of five.

Body Wall: The upper layer of skin is single layered and ciliated. Endoskeleton is of calcareous plates in the dermis which are mesodermal in origin.

Body Cavity: it has a true enterocoelic coelom.

Ambulacral System (= Water Vascular System): it is the characteristic feature of echinoderms that it has ambulacral system. A perforated plate called madreporite is present in this system. The pores of the madreporite allow water into the system Tube teet of this system aids in locomotion, capture of food and respiration. Water vascular system is of coelomic origin.

Digestive Tract: It is complete. Brittle stars have incomplete digestive tract.

Haemal and Perihaemal Systems: Instead of blood vascular system, there are present haemal and perihaemal systems which are of coelome ongin. The circulatory system is of open type and includes haemal and perihaemal systems. The blood is often without a respiratory pigment. There is no heart.

Respiratory Organs: Gaseous exchange occurs by dermal branchae or papulae in star fishes peristominal gills in sea urchins, genital bursae in brittle stars, and cloac respiratory ‘trees in holothnrians. Exchange of gases also takes place through tube feet.

Excretory Organs: Specialized excretory organs are not present. Nitrogenous wastes are diffused out via gills. Ammonia is chief excretory substance.

Nervous System: It consists of a nerve ring and radial nerve cords. Brain is absent.

Sense Organs: sense organs are poorly developed.

Sexes and Fertilization: Except a few individuals, the sexes are separate. sexual dimorphism is absent. Fertilization is external.

Asexual Reproduction: Some forms reproduce asexually by self-division.

Autotomy and Regeneration: Phenomena of autotomy and regeneration are often well marked in echinoderms.

Development: The development is indirect and includes a ciliated, bilaterally sym­metrical larva that undergoes metamorphosis to change into the radially symmetrical adult.

Unique Features of Echinoderms:

1. Presence of spines and pedicellariae.
2. Ambulacral system (water vascular system),
3. Haemal system,
4. Mesodermal endoskeleton of calcareous plates,
5. Bilateral symmetry in the larva and radial symmetry in the adult.

Degenerate Characters:

Lack of head, Simple sense organs,Incomplete digestive tract in some forms,Reduced circulatory system, Absence of excretory system.

Resemblance with Chordates:

Radial and indeterminate cleavage, Gastrulation by invagination, Mouth derived as an ectodermal invagination, Enterocoelous coelom. Both are deuterostomes.

From these resemblances, it is clearly proved that the Echinoderms are nearer to the Chordates than any other group. It also indicates that the chordates have been evolved from Echinoderm-like ancestors.

The phylum Echinodermata is divided into five classes

Class 1 Asteroidea

1. Body is flattened star shaped with five arms
2. They possesses tube feet with a sucker
3. Presence of calcareous plates and movable spines.
4. Respiratory organ: papulae
5. Examples: Asterias (Star fish), Astropecten, Zoraster, Oreaster

Class 2 Ophiuroidea

1. Body is flat with pentamerous disc
2. They possess a long arm which is sharply demarcated from the central disc.
3. They possesses tube feet without sucker
4. intestine are absent
5. Respiratory organ: Bursae
6. Examples: Ophiderma, ophiothrix, Astrophyton, Amphuria, etc

Class 3 Echinoidea

1. Body is disc-like hemi-spherical
2. They are devoid of arms or free-rays.
3. They possesses tube feet with a sucker.
4. They possess compact skeleton and movable spines.
5. Examples: Echinus (Sea urchin), Cidaris, Arbacia, Echinocardium. Diadema

Class 4 Holothuroidea

1. Body is elongated in the oral-aboral axis and it is like cucumber.
2. They have no arms, spines and pedicellariae.
3. The tube feet are sucking type which is modified into tentacles and form a circle around mouth.
4. Respiratory organ: cloacal respiratory tree
5. Examples: Cucumaria (Sea cucumber), Holothuria, Mesothuria, etc

Class 5 Crinoidea

1. Body is star-shaped
2. Some of the forms were extinct and living forms.
3. Arms bifurcated, with two pinnules.
4. They have tube feet without suckers
5. Examples: Neometra, Antedon, Rhizocrinus, etc