Echinoderms

The echinoderms are animals that inhabit the sea whatever region of the world or level of depth. There are more than 7000 species that live free or fixed at the bottom of the sea.

Characteristics of echinoderms

The pentarradial symmetry in adult organisms echinoderm is a distinguishing feature that determines the shape of the body. This aspect is observed quite clearly in starfish whose body has five exactly the same points that are located around a central axis, perpendicular to the points. This physical characteristic makes it difficult to identify the organs of echinoderms. On the other hand, it is pertinent to note that this pentarradial symmetry is bilateral in larvae.

Another particular characteristic is its aquifer apparatus, which is made up of a vessel located in a ring around the esophagus and communicates with the outside of the hydraulic channel. This system allows locomotion, food transport and gas exchanges. On the other hand, they can move thanks to their tube feet, that is, small feet located in the lower part of their body.

Classification of echinoderms

According to their morphology, we can classify echinoderms into the following types:

• Crinoids: popularly known as sea lilies for their branched shape, similar to that of some plants.
• Asteroids: Their body is generally flattened, thin and fragile in appearance. They have a central axis and five arms. They are known as starfish.
• Ophiuroids: Like asteroids, they have five arms but more robust than those of starfish and they do not connect with each other.
• Echinoids: They are more voluminous than the echinoderms of the other classes. They are balloon-shaped and do not have arms. They are known as sea urchins and one of their peculiarities are the thorns that cover their body.
• Holothuroids: Their body is elongated and although they lack arms, their organs are constituted in their internal part, in multiples of five.

Reproduction in echinoderms

On the one hand, the reproduction of echinoderms is generally sexual , of individuals of separate sex and with external fertilization. Embryos develop indirectly and go through different stages as larvae before becoming an adult organism.

On the other hand, reproduction can also be asexual , commonly reproducing themselves by fragmentation.

Feeding of echinoderms

The diet varies according to the type of echinoderm. Starfish are predators and move slowly to catch prey such as mussels or oysters. Digestive enzymes are responsible for processing the tissues of the dam and the liquid produced is sucked inside the digestive tract of the star of the sea.

The urchins , of the echinoid class, feed on the algae of the littoral zone of the oceans. The members of this class have a particular structure, Aristotle’s lantern , located in the mouth and made up of about forty ossicles that form five calcareous teeth joined by muscles. This structure serves to crush the vegetable matter.

For their part, sea ​​cucumbers are benthic and feed on particles that settle . They are suspensivores. They have tentacles around the mouth that extend on the surface of the sediments. The sedimenting particles stick to the mucus that covers the tentacles, and the latter are regularly introduced into the digestive tract, which allows the animal to ingest the collected particles and to coat the mucus tentacles again.

Breathing

The epidermis of echinoderms have ciliated dermal gills in which a countercurrent exchange takes place that allows a greater diffusion. The tube feet also form an exchange surface. The internal tissues, however, are not in direct contact with seawater . The aquifer system, whose internal surface is ciliated, serves as a circulatory system that allows oxygen, nutrients and waste to pass from the epidermis to internal tissues or vice versa.

Habitat

Echinoderms are species that belong only to the marine world since they are unable to regulate their osmotic composition. The larvae of echinoderms live freely in the sea as part of the zooplakton, while adult organisms usually inhabit the benthic areas of the sea, that is, in the deepest parts, in which they burrow or not, in the sandy bottoms or on the rocky seabed.

Examples

Examples of echinoderms include starfish from the asteroid class, sea lilies from the crinoid class or cucumbers from the holothurian class.