Echinoderms (Phylum Echinodermata ) are marine animals that are slow moving or sessile. They have an endoskeleton formed by calcareous plates and, for the most part, have pointed projections (which depart from this skeleton) and spines. One of the most striking characteristics of this group of animals is the presence of so-called ambulacral systems. About 7000 species of echinoderms are currently known, with sea stars and sea cucumbers being two of their representatives.
General characteristics of echinoderms
Echinoderms are a group of animals that live in the marine environment and have an internal skeleton (endoskeleton) formed by calcareous plates, which commonly bear thorns or protruding tubercles. Due to this characteristic, many representatives have a spiny or warty appearance , hence the name echinoderm, which comes from the Greek, echin , which means “covered with thorns”, and derma, which means “skin”. The presence of these structures is related, among other aspects, to the animal’s defense.
The head is not present in echinoderms, its body being organized in an oral-aboral axis. The oral region is where the mouth is, and the aboral region is opposite to it.
Echinoderms are triploblastic organisms (they have three embryonic layers : endoderm, mesoderm and ectoderm) , coelom (they have a body cavity, lined by tissue derived from mesoderm, called coelom ) and have radial symmetry.
It is worth mentioning that radial symmetry is observed in adults, but in larvae, bilateral symmetry is observed. Some authors point out that the symmetry is not truly radial in adults, since in starfish, for example, the opening of the aquifer system is not in the center, but more displaced to one side.
The echinoderms also stand out for being a deuterostome group. In these animals, therefore, during their embryonic development , the blastopore gives rise to the anus. This characteristic makes the phylum Echinoderms more closely related to the phylum Chordates than the other phyla of invertebrates.
A striking feature in echinoderms is the presence of an ambulacral system, also called aquiferous vascular system or hydrovascular system, which consists of a series of channels through which a fluid similar to seawater circulates .
In starfish, this system consists of a perforated plate ( medreporic plate or madreporite ) located in the aboral region and which communicates, through the stony channel, with the circular channel present in the central disc. From the circular channel, five radial channels depart, which extend through the arms of the star. From the radial canals arise the lateral canals, formed by a valve and ending in an ampulla and an ambulacral foot, small tubular projections. It is through the madreporite that water flows in and out of this system.
At the time of movement, it is observed that the ampulla contracts, the lateral canal valve closes, and the fluid is forced into the ambulacral foot, which becomes elongated. When in contact with the substrate, the feet release chemical substances that ensure adhesion to the site. To release, another chemical substance is released, but this time with non-stick properties. The tube feet muscles contract, and water is forced back into the ampulla.
The digestive system of echinoderms is complete, except for brittles. Echinoderms do not have organs specialized in excretion, and gas exchanges occur differently in each group, with gill breathing and also diffusion through the ambulacral system being observed.
The nervous system of echinoderms consists of a nervous ring near the oral region, from which radial nerves depart. These animals have separate sexes, and sexual reproduction usually occurs with males and females releasing their gametes directly into the water. Development is indirect , therefore, with the emergence of larvae.
We can divide the current echinoderms into five groups: Asteroidea , Ophiuroidea , Echinoidea , Crinoidea and Holothuroidea . Let’s see more about each one:
- Asteroidea : Starfish and sea daisies are included. Sea stars, the best known representatives, are formed by a central disk from which arms depart. In the lower region of the arms, it is possible to notice the presence of ambulacral feet, which ensure the movement of the animal and also help in capturing prey.
A curious fact is the way starfish feed. These animals use their ambulacral feet to grab prey, usually bivalve molluscs , and then throw their stomach out, entering the body of the bivalve through small openings between the shells. The starfish then begins to release digestive juices into the mollusk, starting the animal’s digestion inside its own shell. Subsequently, the star retracts its stomach, taking the partially digested food, and completes digestion.
Another important characteristic of sea stars is their ability to regenerate, and it is possible to observe the regeneration of lost arms and even the entire body based on a single arm, as long as the central disk remains intact.
- Ophiuroidea : we have as representatives the so-called serpent-stars. They resemble starfish, but have much longer and more flexible arms. Its movement is made, mainly, by the movement of the arms in a way reminiscent of that performed by serpents. In this group, tube feet are generally not used for locomotion. Ophiuroids do not have an anus.
- Echinoidea : we have as representatives the sea urchins and the beach wafers, which have, respectively, spherical and flattened bodies. These animals, unlike those presented, do not have arms, but have five rows of ambulacral feet.
The movement of these animals is slow, and, in sea urchins, it is done using the tube feet and the spines present on their body. In sea urchins, the presence of a scraper device with five large calcareous plates is observed, which is called Aristotle’s lantern. This structure allows urchins to scrape algae off surfaces.
- Crinoidea : you can find the sea lilies and sea feathers. The lilies remain adhered to the substrate by means of a peduncle, while the feathers manage to move using long arms. The arms of these species can be used for food.
- Holothuroidea : we have as representatives the sea cucumbers, animals quite different from the rest of the echinoderms. Its body is elongated, hence the reason for the name sea cucumber. In these animals, there are five rows of tube feet and tube feet modified in the form of tentacles around the mouth.