Kareena lohia: ECHINDERMATA

PHYLUM- ECHINODERMATA

The phylum echinodermata includes the starfish, brittle star, sand dollars, sea urchin, sea cucumber, and sea lilies. They possess the following characters :-

1. Body from :- The body form has various shapes - star-like, cylindrical, melon-like,disk like, or flower like. It is unsegmented.

2. Symmetry :- The symmetry is radial in the adults and bilateral in the larvae. The radial symmetry of the adult is pentamerous, body parts are arranged in five or multiples of fives. The radial symmetry has been secondarily derived from the bilateral ancestral form, and the echinoderms are in no way related to the radiate phyla, namely, porifera and cnidaria. The echinoderms have a much higher level of structure than the other radiate groups.

3. Germ layer :- The echinoderms are triploblastic animals.

4. Level of organisation :- The echinoderms have organ system level of organisation. All systems have radial arrangement.

5. Head :- The body lacks head in all echinoderms.

6. Appendages :- Peculiar tube feet (podia) are developed for locomotion. The Tube feet generally protrude out through special radial areas called ambulacra. They are extended and retracted by variation in hydraulic pressure of fluid in them and by contractions of their muscles.

7. Body wall :- Epidermis is single -layered and ciliated. It overlord a thick dermis containing mesodermal endoskeleton of calcareous plates. The latter often have spines, hence echinodermata muscles are smooth and lie below the dermis.

Minute pincer-like structures, called pedicellariae, project through the skin in many debris and organisms by opening and closing suddenly.

8. Body cavity :- A true coelom lined by ciliated mesothelium is present. It is enterocoelous, and contains a fluid with free amoeboid cells called coelomocytes. The coelomic fluid plays an important role In food and gas transport.

A part of the larval coelom is modified into a unique water filled ambulacral, or water vascular system with tube feet to help locomotion. A perforated Plate, termed madreporite, permits entry of water into the ambulacral system.

9. Digestive Tract :- The digestive tract is usually complete. It is complete in brittle stars.

10. Respiration :- Respiration occurs by gills called dermal branchiae, or papulae in starfishes, by genital warts bursae in brittle stars, by peristomial gills in sea urchins, by cloacal trees in sea - cucumber, and by tube -feet in all.

11. Circulatory :- The circulatory system is greatly reduced and is of open type. It is called the haemal system. Blood often lacks a respiratory pigment. There is no heart or pumping vessel. The circulatory system has little role in gas transport.

12. Excretory system :- There are no excretory organs. Nitrogenous Waste is ammonia (ammonotelism). It diffuses out by gills, bursae, respiratory trees and tube feet.

13. Nervous system :- The nervous system includes a nerve ring and radial nerve cords. There is no brain. Sense organs are simple.

14. Reproduction :- Saxes are separate. Males and females are extremely alike. Copulation does not occur. Fertilisation is generally external. Some forms reproduce asexually by self diffusion.

15. Development :- Life history includes a ciliated, bilaterally symmetrical larva, that undergoes conspicuousness in brittle stars, echinopluteus in sea urchins, auricularia in sea - cucumber and crinoidea in feather star.

Unique features

The echinoderms have the following characters :-

Bilateral symmetry in the larvae and radial symmetry in the adult.
Demarcation of the body surface into alternating ambulacral and interambulacral areas.
Mesodermal endoskeleton of calcareous plates, usually vascular system for aid in locomotion.
Modification of a part of the coelom into a water vascular system for aid in locomotion.
Characteristic tube feet for locomotion.
Peculiar pedicellariae for cleaning the body surface.

CLASSIFICATION

The phylum EChinodermata is divided into five classes :

Asteroidea
Ophiuroidea
Echinoidea
Holothuroidea
Crinoidea

Class 1. Asteroidea

The astrodea have following characters :-

Body is flat and star like
Arms are not sharply marked off from the central disc

Spines are short.
Oral surface bears mouth and ambulacral grooves.
Aboral surface has annus and madreporite.
Pedicellariae, when present, are small
Viscera extends into the arms.
Larva is bipinnaria or brachiolaria.

Example :- Asterias - The Starfish

Class 2 Ophiuroidea

The ophiuroidea have following characters :-

Body is flat and star-like .
Spines are short.
Arms are sharply marked off from the central disc.

Oral surface has mouth and madreporite.
Annus and ambulacral grooves are absent.
Pedicellariae are absent.
Larva is pluteus.

Example :- ophiothrix - the brittle star, astropecten - the basket star.

Class 3. Echinoidea

The echinoidea have following characters :

Body is globular or disc-like.
There Are no arms.
Spines may be small or large.
Oral surface bears the mouth. The mouth is fitted with a characteristic biting and chewing toothed apparatus called Aristotle's lantern.
Aboral surface carries annus and madreporite.