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8.6. Echinoderms

Phylum Echinoderms

The organisms of the Phylum Echinoderms are  exclusively marine invertebrate animals, with a complex organization. They are beings that have evolved by simplifying themselves by adapting to life at the bottom of the sea.
Echinoderms have an internal skeleton (dermoskeleton) made up of calcareous plates covered by skin, from which spines can protrude . In fact, echinoderm comes from the Greek, ekhino, "spike" and derma, "skin", so it means skin with spikes or spines .
Its main characteristic is that of having a pentaradial symmetry that gives it a star shape. This symmetry is unique to echinoderms and only appears in adult individuals. Due to this symmetry, some organs, such as the head, are not distinguished. In larvae, on the other hand, symmetry is usually bilateral. Radial symmetry has arisen as a consequence of the little or no mobility of echinoderms.
Another exclusive characteristic of echinoderms is their ambulacral system, formed by a network of interconnected tubes and channels filled with water that run through the entire organism. It is involved in the exchange of gases, in feeding, in receiving stimuli, and in locomotion, since they have tube feet, small tubes ending in suckers that they use for movement.
Echinoderms can be globose, like sea ​​urchins, elongated, like holothurians or sea ​​cucumbers, or star -shaped like starfish and brittle stars.
Echinoderms have a complete digestive system, with a mouth with five radial teeth that form Aristotle's lantern, the chewing apparatus. The mouth is at the bottom of the body and the anus at the top. Their diet is based on algae and small crustaceans.
The reproduction of echinoderms can be:
  • Asexual, by fragmentation. A starfish, for whatever reason, can split in two and, from each fragment, a new individual can emerge. For this, it is necessary that each of the fragments contain part of the central disk of the star.
  • Sexual. Normally individuals have separate sexes and fertilization is external. They are oviparous. The development of the embryos is indirect, and the larvae have to go through several phases until they become an adult individual.

The phylum echinoderms is divided into two subphyla:
  • Subphylum Pelmatozoa: they are immobile.
    • Class Crinoids .
  • Subphylum Eleutherozoa: They are mobile.
    • Class Asteroids.
    • Class Ophiuroidea.
    • Class Echinoids.
    • Class Holothuroidea.

Class Crinoids

Crinoids are also called crinoids or sea ​​lilies, from the branched appearance of their arms, or "feathered stars", and is the most primitive group of echinoderms.

The pedunculated crinoids live fixed to the bottom of the sea, united by a calcareous peduncle. The rest of the crinoids lack a peduncle and move very slowly along the seabed. The body has the shape of a calyx or cup, and five highly branched articulated arms covered with spikes protrude from it. In the center of these five arms is the mouth with which they filter food from the water.

Unlike the rest of the classes of echinoderms, the crinoids have the mouth and the anus on the upper face.

Esquema de un crinoideo

By Enciclopedia Britanica (Wikimedia commons) [CC BY-SA 3.0 ], via Wikimedia Commons

Asteroids class

Asteroids are a class of echinoderms characterized by the presence of five arms attached to a central disk, usually with a flattened body covered by many spines. Its exoskeleton is formed by the fusion of calcareous plates.

They are commonly known as starfish. Although the image we have of the starfish has five arms, in some species they are so short that they cannot be distinguished and, in others, they can have between 7 and 20 arms, as occurs in sun starfish.

In the ventral part (below) of the arms are the tube feet that, as in the rest of the echinoderms, are part of the ambulacral system that intervenes both in circulation, and to move or fixate on the ground.

The dermoskeleton plates are articulated and the spines are shorter, which differentiates them from echinoids.

In the ventral part there is also the mouth, where a short digestive tube begins, with a very developed stomach and an intestine with many ceca that ends in the anus, in the dorsal part of the body.

Its reproduction is sexual, with external fertilization. Sometimes, the starfish has to regenerate some part that has been sectioned, but in addition, another new individual can arise from the fragment (asexual reproduction).

They are carnivorous, feeding on other invertebrates (molluscs, other echinoderms, etc.) that live in benthic zones (at the bottom of the sea).

Pies ambulacrales de una estrella de mar

Class Ophiuroidea

Ophiuroids are very much like starfish (asteroids), they also have five arms but they are much thinner than those of starfish and are not attached to each other.

The class ofiuroideos is formed by brittle stars or serpent stars.

The body of brittle stars has a central disc in the form of a pentagonal plate from which five cylindrical arms, simple or branched, emerge, which they use to move.

They do not have an anus, so they feed and defecate through their mouths. Tube feet, if any, are very small. The arms of brittle stars are very fragile and break easily, but they can easily regenerate.

They are carnivorous beings.

Ofiura en el fondo del mar

By GrahamBould [Public domain], from Wikimedia Commons

Class Echinoids

The organisms of the Class Echinoids are known as sea ​​urchins.

The body of sea urchins is balloon-shaped and covered with spines, which is why they are called urchins. Unlike other echinoderms, they do not have arms. They have a shell formed by fused pentagonal calcareous plates of great hardness, so many of them can fossilize.

The mouth is in the ventral part of the organism, which gives rise to the digestive system that has a large stomach. Next to the mouth is Aristotle's lantern, a structure of five teeth with which it breaks food. At the other end of the body is the anus.

They have tube feet and an ambulacral system that allows them to move.

They usually live among the rocks of the coastal area.

They are herbivores, they feed mainly on algae, but they can also feed on the remains of living beings (ghouls) and other foods.

Its reproduction is sexual.

Erizo de mar en el fondo del mar

Kirt L. Onthank [CC BY 3.0 ], from Wikimedia Commons


Class Holothuroidea

The Class Holothuroidea are known as holothurians or sea ​​cucumbers. 

The body of sea cucumbers is soft, cylindrical, and divided into five radial zones that contain the tube feet.

Sea cucumbers do not have arms or spines, like other echinoderms.

Although they appear to have bilateral symmetry, internally they retain pentaradial symmetry in the internal distribution of their organs.

The skeleton of holothurians is very small and is made up of scattered, segmented calcareous plates.

The holothurians live in the bottom of the seas (they are benthic beings). They dig in the sand and feed on organic remains, helping themselves with the branched and flexible tentacles that they have next to their mouths. They are detritivores.

Reproduction is sexual, by separate sexes, and fertilization is external.

Pepino de mar sobre el fondo del mar
By Jacinta Richardson and Paul Fenwick [GFDL or CC-BY-SA-3.0], from Wikimedia Commons