1 E-8 S

The echinodermatous , whose name means with thorny skin in Greek, form a very old group of marine animals, whose first fossil traces go back to the Cambrien. It counts 7.000 species living currently, whose most known representatives are the sea urchin S, and 13.000 which disappeared. Very original, they have certain a number of single characteristics in the animal world. The principal ones are a pentaradiée symmetry and the presence of a aquiferous Système. Group brother of the Chordés within the Deutérostomiens (certain minority scientists give this classification causes some to arrange them among chordés).

Morphological characteristics

Echinodermata are marine animals Benthique S present at all the oceanic depths.

The adult form is organized around a pentaradiée symmetry, but the embryonic forms - bilateral - arrange at the sides of twisted in the group of the deutérostomiens. Besides they share with them good number of biochemical characteristics. This symmetry is found in general morphology but also in the whole of the organization of the body, since the nervous system to the calcified skeleton made up of plates and spines which surround and protect the interior of the body.

Their vascular system is reduced and open. At side they have an aquiferous system which ensures some of the functions of the Sang and some others: gaseous exchange, locomotion and nutrition. However the digestive system is complete and perfectly functional. The Nervous system consists of a nervous ring around the mouth from which leave the nerves according to a radiate structure (not of central nervous system).

The sexes are separate. The Sperm and the Ovocyte S are released in the medium, the Fécondation takes place in the external medium randomly. Echinodermata, and particularly the sea urchin S were very useful in the comprehension of the phenomenon of fecundation, in particular with regard to the ionic movements during the penetration of the Spermatozoïde.

Phylogenetic origin

The first attested traces Fossile S go back to 700 million years to the Cambrien. They derive from forms with bilateral symmetry, the hétérostèles. Pentaradiée symmetry was not essential immediately, a Precambrian form of Ediacara ( tribrachidium ) is indeed triradiée, but its membership of Echinodermata is discussed. The oldest forms are fixed and include/understand the cystoïdes and the blastoïdes. Badly encircled, is the carpoïde (taxon the mitral ones), a strange form that some scientists classify among twisted.
Thereafter, Echinodermata left abundant traces, in particular in the group of the Crinoïde S and the echinoid which is present as of the Ordovicien. On the whole 13 000 Espèce S extinct were identified (against 7 000 today).

Classification

Traditional classification

According to ITIS:

subphylum Eleutherozoa Bather, 1900

super-class Asterozoa Zittel, 1895

class Asteroidea of Blainville, 1830 (starfish)
class Somasteroidea Spencer, 1951

super-class Cryptosyringida Smith, 1984

class Echinoidea Leske, 1778 (sea urchin)
class Holothuroidea of Blainville, 1834 (Cucumber of sea)
class Ophiuroidea Gray, 1840 (Ophiure)

subphylum Pelmatoza Bather, 1900

class Crinoidea Miller, 1821 (Crinoïde)

Clean characters of the group

the larva is with bilateral symmetry .
They are sometimes fixed at a substrate at the adult stage, and then gave up the mobility of the Bilatérien S.
radiate symmetry, generally of order five (pentamer), is acquired at the adulthood.
Their tegument is roughcast prickles, or is reinforced spicules or plates.

Clean derived characters

the internal skeleton (when there exists) consists of calcite monocrystals.
a system ambulacraire allows the movement, by play of differential pressure in the feet ambulacraires.

Place des Echinodermata in the animal kingdom

Phylogenetic classification

Gallery of images

Click on the titles to reach other images as of these 5 principal orders of Echinodermata:

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