General characters of Phylum Echinodermata
There are more than 5000 known species of Phylum Echinodermata (Echinoderms). They show following characters:
- Habitat: The echinoderms are exclusively marine. Most of them are present in lie bottom along shoreline in shallow sea. Most species are free living. However, some are attached to the substratum.
- Body wall: The body is covered by a delicate membrane. The mesodermal cells develop a firm calcareous exoskeleton. This endoskeleton may spring of epidermis. This exoskeleton is mesodermal origin. So it may be called endoskeleton.
- Body from: They are triploblastic and coelomate. The mouth is present on the lower or oral surface. The anus is present on the upper or oral surface.
- Symmetry: The larvae of echinoderms show bilateral symmetry. But the adult show radial symmetry. It’s an adaptation for their mode of life.
- Shape: The body may be:
- Flattened like biscuit: Cake urchin.
- Star shaped with short arms: Starfish.
- Star shaped with long arms: Brittle star.
- Globular: Sea urchin.
- Elongated: It has no central disc from which arms radiate: Sea Cucumber.
- Water Vascular System: It is their unique characteristic. Water vascular system is present in the coelom. It is complex system of tubes and spaces. It surrounds the mouth and passes into arms and tube feet. Water enters these canals through a sieve-like plate called madreporite. Water circulate thorough these channels. Water canal system plays a role in locomotion. It makes the tube feet erect for attachment with the substratum.
- Locomotion: The free living species move with the help of tube feet. Each tube foot is a soft sac like structure. It is present along the edge of grooves in the arms.
- Organization: The echinoderms show low degree of organization. There are specialized organs for digestion and reproduction. But there are no specialized organ for respiration and 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 developed.
- Reproduction: The sexes are separate and the fertilization is external. They develop larvae like bipinnaria larva and brachiolaria larva. These larvae have complex structures and they show bilateral symmetry. These larvae resemble to the larvae of chordates,
- Regeneration: The ability to form lost organ is called regeneration. It is common in echinoderms like starfish, sea cucumber, sea lily, brittle star and sea urchin.
Examples: Asterias (Starfish) Sea urchin, Sea cucumber, Cake urchin and Brittle star.
Position of the Echinodermata
The echinoderms are comparatively simple in structure, organization and physiology. So they deserve a place slightly below the annelids. But echinoderms show striking resemblances with the Chordates. So they are placed at the top of the lists of the invertebrate phyla. There are following resemblances between echinoderms and chordates:
- There is a radial cleavage during the development of embryos in both phyla.
- The blastopore forms the anus in both phyla.
- There are certain common biochemical properties both in echinoderms and chordates. For example, phosphocreatine (an energy storing compound) ¡s present in both phyla.
Affinities (relationships) of Echinodermata
Echinodermata do not show close relationship with most of the invertebrates. They show affinities with Hemichordata.
Resemblances with Hemichordata
Both these groups show following common features:
- In both cases, the blastopore forms the anus. While ¡n all other invertebrates, the blastopore forms the mouth.
- There are similarities in the formation of coelom. In both groups, the mesoderm is derived from the cells close to the blastopore.
- Both possess mesodermal exoskeleton. While the exoskeleton of other invertebrates are ectodermal in origin.
These resemblances between Hemichordata and Echinodermata are not accidental. These resemblances are also not developed due to convergent evolution (two different groups start resembling each other). Actually, they show these resemblances because they are closely related and have emerged from the same (common) ancestor
Resemblances with Chordata
Echinoderms also show following very close resemblances with chordates:
- Both have mesodermal exoskeleton.
- Both are deuterostomous.
- In lower chordate and echinoderms, the early development is almost similar.
Due to these similarities, the Echinodermata have been placed closet to phylum Chordata.