Echinoderms are marine animals which are typically characterized by their radial symmetry and water vascular system. They neither have a brain, nor any complex sensing organ. So, how do they manage to carry out all life tasks, which we do with the help of our brain and sensory organs? In order to understand that, you need to get a good understanding of their general characteristics.
In marine biology, those species which belong to phylum echinodermata are known as 'echinoderms'. Interestingly, the name echinodermata is derived from a Greek word meaning 'spiny skin'. This phylum is believed to have been existing on the planet since the Cambrian period. It comprises more than 6000 marine species, including starfish, brittle stars, sea urchins, and sea cucumbers. Echinoderms are found in all the oceans of the world at different depths, with major concentration being observed in the vicinity of the coral reefs.
Characteristics of Echinoderms
With over 6000 species making up this phylum, it isn't surprising that it's quite diverse in nature. Basically, their characteristics can be broadly categorized into two groups, general, which are common to all the species, and unique, which are restricted to a few species.
One of the most basic traits on the basis of which you can identify echinoderms, is their radial symmetry. All the species are typically characterized by a central structure surrounded by 5-part symmetry. This trait can be easily observed in starfish and brittle stars. Even though it is not so obvious in sea urchins and sea cucumbers, a closer examination of their anatomy shows that even they have a radial symmetry.
Yet another common trait observed in these species, is their internal skeleton covered with spines and skin. This, however, tends to differ from one species to another. In sea cucumbers, the skeleton is made of degenerated calcareous plates, buried in the fleshy body. Similarly, the anatomy of the starfish reveals that its skeleton is made from movable calcareous plates, thus forming flexible joints.
The third, but the most important characteristic trait is their water vascular system. It is most important, because almost all the functions of these creatures, including locomotion, respiration, and feeding, are facilitated by this process. The water vascular system is basically a hydraulic network of canals, which runs through their body. They carry out various life functions by varying the internal water pressure in this hydraulic network.
Even though most of the echinoderms are benthic in nature, i.e., they live on ocean floor, there do exist some, which have the tendency of floating (sea cucumbers) and swimming (sea lilies). Similarly, there also exist echinoderms who take help of some floating debris (crinoids) or fish (sea cucumbers) to move from one place to another. While the 5-part symmetry is very common, some species of starfish and crinoids exhibit a radial symmetry which is in multiples of five. The Helicoilaster (a type of starfish), for instance, possesses up to 50 arms.
Owing to their tendency to flourish in the dark depths of oceans, which are deficient in biotic factors, these sea creatures are very important for the biodiversity of our planet. These species also play a crucial role when it comes to geological environment of the planet, as they contribute to various geological processes at the ocean floor; limestone formation being one of the best examples of the same.
As far as their relationship with humans is concerned, some of them are harmful and some of great help. While sea urchins have numerous spines, which can pierce human skin and require medical intervention for removal, sea cucumbers are considered delicacies in various parts of the world.