General Characteristics

There are over 5,000 known species of echinoderms. They are marine organisms living at the sea bottom.

The body is covered by delicate epidermis. The mesodermal cells develop a firm calcareous endoskeleton which may bear spines and because of its origin, from mesoderm it is called endoskeleton.

Echinoderms are triploblastic coelomates and exhibit radial symmetry. The mouth is on lower surface (oral) and anus is on upper surface (aboral).

The echinodermata are exclusively marine and most of them are found at the bottom along shorelines in shallow seas. Most species are free moving however some are attached to the substratum.

All the larval forms of these animals exhibit bilateral symmetry but the adults show radial symmetry which is an adaptation for their special mode of life.

The body may be flattened like biscuit (cake urchin), star-shaped with short arms (starfish) globular (sea urchin), star-shaped with long arms (brittle star) or elongated (sea-cucumber). There is a central disc from which arms radiate.

The most unique characteristics of echinoderms is that a water vascular system is present in their coelom. It is a complex system of tubes and spaces surrounding the mouth and passing into the arms and tube feet. The water circulates through these channels. Water enters these canals through a sieve-like plate called madreporite present on the aboral body surface.

The motile species move with the help of tube feet. Each tube foot is a soft sac-like structure present along the edges of grooves present in the arms.

The echinoderms exhibit low degree of organization. There are specialized organs for digestion and reproduction, but there are no specialized organs for respiration or excretion. The nervous system is also poorly developed. There is no brain; however a nerve ring is present around the pharyngeal region. Similarly the circulatory system is poorly organized.

The sexes are separate and the fertilization is external. The larvae such as bipinnaria and brachiolaria are complex, exhibit bilateral symmetry, and resemble those of chordates.

Regeneration, the ability to reform lost organs is common among echinoderms starfish, sea cucumber, sea lily, brittle star and sea-urchin exhibit this characteristics.

The echinoderms are comparatively simple in structure, organization and physiology, and deserve a place slightly below the annelid worms. However, these are placed at the top of the list of invertebrate phyla. This is because there are a number of striking resemblances, between the echinoderms and chordates, such as:

  1. There is radial cleavage during the development of embryos in both phyla.
  2. the blastopore forms the anus in echinoderms as well in chordates (deuterostomes).
  3. there are certain common biochemical peculiarities among echinoderms and chordates e.g.phoshocreatin is present in both.

The common examples are:

Asterias (starfish):

Sea urchin           Sea cucumber

Cake urchin        Brittle star

brittlestar   sea urchin

Examples of Echinoderms.

Echinodermata /Affinities

Echinodermata do not show close relationship to most invertebrates, but they do show affinities with hemichordate. Both these have a number of common features among which are the formation of coelom and retention of blastopore as the site for future anus. In both mesoderm is derived from the cells close to the blastopore. Both possess mesodermal endsheleton where as the exoskeleton is ectodermal in orgin wile in invertebtrates the blastopore develops into mouth.

The above resemblances between two phyla are neither accidental nor due to convergent evolution but are because the two are closely related and both emerged from the same (common) ancestor. Echinoderms also very close resemblance with chordate because both have mesodermal skeleton, are deuterostomous, in both lower chordates and echinoderms the early development is almost similar. That is why they have been placed closest to phylum chordate.