The arthropods ( Arthropoda ) - Greek arthron “articulation” and podos “foot”, so called “articulated” - a animal junction form Invertébré S. the body of the arthropods formed of segments (or Métamère S) is articulated, covered with a rigid Cuticule, which constitutes to them external Squelette, in the majority of the case made up of Chitine.

The junction of the arthropods is by far that which has the most Espèce S and the most individuals of all the Animal kingdom. One counts more than one million and half of current species of arthropods.

Morphology

See also: Anatomy of the arthropods

General characteristics

The arthropods or articulated are:
  • Ringed and metamerized like some towards.
  • They moreover are articulated, and equipped with articulated appendices.
The articulations are made necessary by the presence of Chitine, matter coriace, on the surface of the skin. The articulated appendices adapt to the most various functions.

The general cavity is reduced to a whole of gaps where blood circulates.

The nervous system is ganglionic.

During the development occur driven , and very often of the metamorphoses.

  • joined Appendices (1)
    • There exist uniramés Appendices (3 épipodite S + X? biramés Endopodite) and (2 épipodites + 1 Exopodite + X? endopodites).

  • Corps segmented (2)

    • One of the characteristics of the arthropods is that their bodies are métamérisé, (they are Cœlomates).

  • external Squelette (3)

    • the arthropods has a Exosquelette, these are indeed Cuticulates whose cuticule is segmented in 3 types of segments: tergite (dorsal face), pleurite (side face where the appendices fit) and sternite (ventral face).

Characters due to chitin

The Chitine (of the Greek chitinos , tunic) is a nitrogenized substance secreted by the Ectoderme. Initially thin and flexible, it thickens in a resistant carapace, which revêt not only the outside of the body, but also former and posterior parts of the digestive tract.

One considers chitin as the dominating character to which are subordinates the following characters:

  • articular Membranes. If a carapace of only one part is acceptable at the Némathelminthe S, parasitic animals and little mobiles, it is irreconcilable with the mobility of the arthropods. Also it is stopped, with the limit of the successive rings, by flexible articular membranes.
  • articulated Appendices. In the same way, the appendices, under penalty of being rigid rods and without function, must be made of articulated segments. From there the name comes from the arthropods (of the Greek arthron , articulation, and podos , foot).
  • Absence of cilia.
  • specialized Muscles. Muscles which would link a point with another of the same rigid segment would not be used for nothing. There is not thus, in the arthropods, a continuous muscular layer as in the worms, but of the specialized and individualized muscles.
  • Moults of the arthropods. The increase in body is obstructed by the carapace. The arthropods must thus reject it from time to time. It is the phenomenon of the moults. With each moult, the compressed fabrics dilate, and the secret Ectoderme of news Chitine. It results from it that the growth is discontinuous, instead of being continuous as at the other species of animals.

Appendices

The appendices are composed typically of a base, or protopodite , made of two segments which carry each one two side segments.
  • On the lower segment fits a gnathobase masticatrice on the internal side, and a respiratory épipodite on the external side.
  • On the higher segment fits a endopodite on the internal side, and a exopodite on the external side.
These various parts remain simple only in lower shellfish. In all the other arthropods, they become complicated and adapt to various functions. It results from it from the sensory appendices (antennules, antennas), masticateurs (mandibles, maxillule, maxille, maxillipedes), préhenseurs (grips, Pédipalpe S), locomotors (legs starting from the first 5 pairs of appendices, fins starting from pairs 11 to pairs 16), reproducers (bodies of coupling) starting from the 11th pairs of appendix in the male, etc

See also: Leg of the arthropods, oral Parts of the arthropods, Antenna of the arthropods

Circulatory apparatus

See also: Mark of the arthropods

The circulatory apparatus is a whole of gaps. There is neither capillary S, nor Veine S. the Artère S are themselves reduced.

The Heart, located dorsalement, is made of one or several pockets in linear and bored series each one of a pair of openings or ostioles . To each dilation, blood is aspired through the ostioles. With each contraction, it is driven out in the arteries.

Blood is colorless or bluish (Hémocyanine), and contains only white Globules.

Apparatus excretor

The apparatus excretor is very variable according to the groups. It is made of some glands which perhaps derive from Néphridie S.

Nervous system

The Nervous system is ganglionic and ventral.

There are always a pair of cerebroid ganglia and a pair of ganglia under-œsophagiens connected by a collar périphagien . The remainder of the nervous system is in " scale of corde" or " twist with nœuds".

The ganglia of each pair are linked by a commissure . The successive pairs are linked by connective .

At the higher types, the ganglia tend to amalgamate in one or more large masses.

Eyes

The eyes are simple (ocelle S) or made up (eyes with facets). A made up eye can be formed several hundreds of ommatidies . Each Ommatidie includes/understands:
  • a layer of corneal cells , constituting a transparent Cornea (one of the facets of the eye).
  • a layer of crystalline cells secreting between them a Crystalline oblong.
  • a layer of retinal cells secreting between them Stick S sensitive to the light.
The ommatidies are separate from/to each other by black cells.

It is probable that the made up eyes give to their owner a panoramic sight (broad field of vision), but very vague.

Direction

The arthropods move especially by the touch and the sense of smell, which have their seat in sensory hairs . Those are hollow and contain the prolongation of a sensitive neuron.

There can also be auditive bodies and bodies of balance.

Behavior

Predation

Many arthropods are Prédateur S and are useful for the man by driving out the ravageurs. Of other are Ravageur S. to protect some arthropods use the Mimétisme to see Myrmécomorphisme

See also: predatory Arthropod of the ravageurs

Reproduction

The sexes are generally separate. The eggs , rather charged with vitellus ( eggs hétérolécithes ), have a partial or unequal segmentation.

At the time of the moults often occurs, during the development, of the abrupt changes of form, in other words metamorphosis S.

Principal types of arthropods

Radiative explosion of the arthropods

On the general formula of the worms, the arthropods superimposed several innovations:
  • the segmentation, shared with many other organizations, which consists in lengthening the body by repeating of the same segments anatomy.
  • formation of locomotor legs. Tentacles playing the part of legs are present at certain towards.
  • the transformation of the skin into a rigid skeleton, the exosquelette.
This gaining formula corresponds to the general form of the millepede. It was immediately at the origin of a new radiative explosion, which explored various formulas to transform such or such group of legs into jaws, antennas, specialized legs, or to let them regress in the tail. The key question which seems to have structured its distribution is: how much legs is necessary it to move?

Trilobites: A score of legs?

The trilobites were the kings of the marine world, but with the primary era. It is a now extinct group.

The trilobites are well-known because they are certainly the second groups most widespread of fossils after the dinosaurs. In addition, they gave the most diversified fossils: one counts between nine and fifteen thousand species. The majority of them were simple and small marine animals, which filtered the mud to feed.

The trilobites varied in the face, between a millimetre and soixante-douze centimetres, with an average between two and seven centimetres. Largest trilobite of the world, Isotelus rex, was found in 1998 by Canadian scientists on the shores of Hudson Bay.

Crustacean: Ten legs?

The zoologists indexed approximately 55.000 shellfish species.

Most known are the crab S, Crevette S, Homard S and others Langouste S, all are shellfish Décapodes (with ten legs). The number of legs is however very variable among shellfish.

The Cloporte for example is also a shellfish, a Isopode.

The shellfish are recommended by the nutritionnists because of their important content of Oligo-élément S.

Chélicérates, Arachnida and spiders: Eight legs?

During the primary era, with the Cambrien, there is that approximately 540 million years, whereas the sea grouillait of Trilobite S and that the borders of the primitive forests were populated crawling giants (of the millepede of 1,50 m), appeared the first arthropods with grips of the under-phylum of the Chélicérates, whose majority adapted then to the terrestrial life.

It is at that time that one meets first Arachnida having of the Chélicère S. Later, as of the higher Silurien, we discover species of scorpions and Araignée S extremely similar to those and those which we can find at present.

One to date listed approximately 80.000 species of Arachnida, including more than 1500 species of scorpions and 50.000 species of spiders living in all the existing biotopes, of the tropical areas to the polar regions. The majority of Arachnida are however terrestrial.

The others chélicérates are very marginal, most known is the Limule.

The Insectes have six legs

The most advanced insects (Ptérygote S) are the carriers of a major change: wings. They enable them to conquer the airs, and allied with the simplification which the animation of only six legs brings to move, were at the origin of a third radiative explosion.

It is by the superposition of these three radiative explosions (vermiform, arthropods, insects) that the insects dominate the world by their variety: they represent today nearly 80% of the described animal species.

Classification

Place des arthropods in the animal types

Traditional classification

junction Arthropoda

Phylogenetic classification

With the worms Nematode S and some other groups, they constitute the Ecdysozoaire S.

The arthropods are a group which joins together at the same time tax alive and Fossile S. For reasons with difficulties of classification of the majority of these fossil species, that some paleontologist S place among the arthropods and of other in different phyla, the current Arthropods are joined together in the taxon monophyletic Euarthropodes, included in the taxon of the Arthropodes to the general direction, even placed to him in the vaster clade of the Panarthropodes which also joined together the Tardigrades and the Onychophores.

See too

Related articles

  • Euarthropodes
  • Panarthropodes

External bonds

External bonds

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