Phylum Echinodermata- The spiny skinned animals

Introduction to Phylum Echinodermata: Unveiling Spiny Marvels of the Sea

General Characteristics of Phylum Echinodermata

Grade of organization

Echinoderms are triploblastic coelomates.

Shape and symmetry

They exhibit radial symmetry. The mouth is on the lower surface area (oral) and the anus is on the upper surface (aboral). All the larval types of these animals show bilateral symmetry however the grownups show radial symmetry which is an adaptation for their special mode of life.

Habitat

The Echinodermata are solely marine and most of them are found at the bottom along shorelines in shallow seas. A lot of species are complimentary moving however some are connected to the substratum.

Endoskeleton

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

Body form

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

Spines and Pedicellariae

Numerous echinoderms bear spines and pincer-like pedicellariae. The spines provide protection to organisms. The pedicellariae keep the body surface clear of particles, dust and debris, and minute organisms.

Water Vascular System

The most special characteristic of echinoderms is that a water vascular system is present in their coelom. It is a complex system of tubes and areas surrounding the mouth and entering 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.

Organs and Organ Systems

They have a low degree of organ organization.

Nervous system

The nervous system is improperly developed. There is no brain, however, a nerve ring is present around the pharyngeal region. Similarly, the circulatory system is improperly developed.

Digestive system

There are special modified organs for food digestion.

Excretory and Respiratory System

There are no specialized organs for respiration or excretion.

Reproduction in Phylum Echinodermata

There are specific organs for reproduction. 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

Regeneration, the capability to reform lost organs, and is common among echinoderms, starfish, sea cucumber, sea lily, brittle star, and sea-urchin display this characteristic.

Echinodermata and resemblance with Hemichordate and Chordata

Echinodermata do not show a close relationship to the majority of invertebrates; however, they do show affinities with hemichordata. Both these have a number of common features among which are the formation of coelom and retention of blastopore as the site for the future rectum. In each mesoderm is originated from the cells close to the blastopore. Both have mesodermal endoskeleton whereas the exoskeleton is ectodermal in origin while in invertebrates the blastopore develops into the mouth.

The above similarities in between two phyla are neither unintentional nor due to convergent evolution however, are since the two are closely related and both emerged from the very same (common) ancestor. Echinoderms likewise reveal very close similarities with chordates since both have a mesodermal skeleton, are deuterostomes, in both lower chordates and echinoderms the early development is almost similar. That is why they have actually been put closest to phylum Chordata.

The echinoderms are relatively simple in structure and physiology and deserve a location slightly listed below the annelid worms. However, these are put at the top of the list of invertebrate phyla. This is due to the fact that there are a number of striking similarities, in between the echinoderms and chordates, such as:

1. There is radial cleavage throughout the development of embryos in both phyla.
2. The blastopore forms the anus in echinoderms as well in chordates (Deuterostomes).
3. There are certain typical biochemical peculiarities among echinoderms and chordates e.g. phosphocreatine is present in both.

Classification of Phylum Echinodermata

The phylum Echinodermata is divided into five classes.

Class Asteroidea (aster- star, eidos- type)

The body is star-like. 5 arms are generally present which are not greatly marked off from the central disc. Larval kinds are Bipinnaria and Brachiolaria.

An example is Asterias (Starfish).

Class Ophiuroidea (Ophis- snake, Oura- tail, eidos- type)

The body is star-like. Arms are greatly marked off from the main disc. Ambulacral grooves” are missing. Pedicellariae are absent. The larval form is Ophiopluteus.

An example is Ophiothrix (brittle star).

Class Echinoidea (echinos- hedgehog, eidos- type)

The body is globular or dislike. Biting and chewing device with teeth called Aristotle’s Lantern exists. Ambulacral grooves are absent. Larval forms are Platens and Echinopluteus.

Examples are Echinus (sea-urchin), Echinarachinus (sand dollar).

Class Holothuroidea (Holothurion- sea cucumber, eidos- form)

The body is extended and round. The oral end has a mouth surrounded by arms. Ambulacral grooves are absent. Spines and pedicellaria are absent. Larval forms are Auricularia and Doliolaria.

Example: Holothuria (sea cucumber).

Class Crinoidea (Crinon- lily, eidos- form)

Body has a main disc which is connected to the substratum. Arms are branched. Spines and pedicellariae and madreporite are absent. The larval type is Doliolaria. They are typically called feather stars or sea lilies.

Example: Sea lilies.

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