Phylum Echinodermata

‘Echinodermata’ means ‘spiny skinned.’ There are approximately 6000 living species belonging to this phylum, and Echinodermata has characteristics that are unique in the animal kingdom. There are five major classes of echinoderms: class Ophiuroidea (brittle stars), class Asteroidea (sea stars), class Crinoidea (sea lilies), class Echinoidea (sea urchins), and class Holothuroidea (sea cucumbers). Echinoderms belong to the Deuterostomia branch of the animal kingdom. Deuterostomes have their anus developing from or near the blastopore, and the mouth is elsewhere. They have a coelom, which has budded off the archenteron, radial and indeterminate cleavage, and endomesoderm from enterococci pouches.

General characteristics

Some of the main characteristics of echinoderms are listed below. 

  • Their body is not metameric; it has radial, pentamerous symmetry, which means they can be divided into symmetrical parts from a central point (just the way a pizza is cut!). 
  • They have an internal skeleton made up of bony plates of calcium carbonate.
  • They have a water-vascular system of coelomic origin that extends as a series of tentacle-like projections (podia or tube feet).
  • The spines are part of the internal skeleton covered by the epidermis.
  • They generally cannot move fast, except the deep-sea Holothuroids.
  • They cannot osmoregulate and hence are rarely found in brackish waters.
  • They do not have a brain and have few specialized sensory organs.
  • Hemal systems are present and excretory organs are absent.
  • They begin as bilateral free-swimming larvae and undergo metamorphosis into a radial adult.

Classes of echinoderms

There are five major classes of echinoderms: class Asteroidea (sea stars or starfish), class Ophiuroidea (brittle stars), class Echinoidea (sea urchins), class Holothuroidea (sea cucumbers), and class Crinoidea (sea lilies or feather stars).

Class Asteroidea

There are approximately 1700 living species of echinoderms belonging to this class. They move with the help of hundreds of tube feet, which are tipped with a suction disc (powered by the water-vascular system). The water-vascular system has a central ring canal and tubes running out into each ray. The tube-foot is connected to the radial canal and a balloon-like ampulla, which works like a squeeze bulb, forcing water in and out so that the foot can straighten or extend. They have eyespots at the end of each arm that are sensitive to light. They feed by forcing their stomachs out and onto the prey. The partially digested nutrients are sucked back. There are five gonopores on top that shed eggs or sperms into the sea.

Class Ophiuroidea

‘Ophiuroidea’ means snake-like. These species can have their arms broken off, only to be regenerated! There are over 2000 species of this class of echinoderms. They have pointed tube feet, and they move by using their arms in a rowing motion. Their mouth is on the underside of the disc. They eat small pieces of decaying matter near the seafloor.

Class Echinoidea

These species have a rigid endoskeleton with a covering of outward-pointing spines. They are primarily herbivores, and their mouth is on the underside. They have a unique jaw structure called the ‘Aristotle’s Lantern’. The sand dollars live in coarse sand, while the heart urchins live in soft mud.

Class Holothuroidea

There are over 900 species that belong to this class of echinoderms. They are detritivores and eat dead, decaying matter near the seafloor. They have a worm-like body and tube feet. They do not have a bulky endoskeleton, and their skin is rather leathery. As defence, they secrete powerful toxins from the skin and have toxic sticky tubes protruding out of the anus.

Class Crinoidea

These are found in warm, tropical seas and are attached to corals and other things. Their mouth and anus are found on the top side of the body, surrounded by five sets of branching, tentacle-bearing arms that trap prey.

Echinoderms are a puzzle to zoologists as they have characteristics that are unique in the animal kingdom. Scientists don’t have all the answers to all the questions regarding echinoderms, and there remains a lot unknown.