Polychaeta

Introduction

Primarily marine animals comprising the greatest number of species at Annélides, with more than 10.000. The reproduction is bisexuée (separate sexes). They are different from the others towards ringed by the presence of a head developed well, by Parapode S, appendices even provided with silks present on the near total of the segments and finally, like by the absence of clitellum. They are divided into two groups, the Sédentaires (fixed at the substrate) and wandering them (capable of movements and thus, to move). The sedentary polychètes are mainly Tubicoles. The wandering polychètes, like the kind Nereis , are the predatory ones provided with jaws or teeth.

Anatomy and biology

Installation of metamerism at the polychètes

Development of the larva Trochophore

The ectoderm form the external wall of the body, it carries lashes which intervene in the locomotion and the nutrition. They are laid out on 2 crowns: the prototroche (around the mouth) and the métatroche. There is also an apical tuft of lashes. The beat of the lashes makes it possible to direct the displacement of the larva and plays the part of rudder.

The endoderm gives the digestive tract, which is bent, it opens by the mouth and the anus. The mésoderme gives mesodermic masses to the back. By cellular division there is a growth of the mesodermic masses and mesodermic formation of strips along the digestive tract. Oldest are pushed back with the front one. The mesodermic mass grows hollow and will form the coelomic blisters. These coelomic blisters are formed by schizocoelie.

The whole of the coelomic blisters form the filled up coelome of coelomic liquid. The size of the larva increases, then becomes métatrochophore.

Metamerism involves the segmentary repetition of the bodies, that relates to only the mesodermic bodies. ectoderm, not metamerized, form a skin of only one layer. It is monostratifié and surmounted by a cuticule. The skin emits 2 side extensions in the shape of sheet which will give the parapodes. The Ectoderm form also the nervous system and the ventral nervous chain. The Endoderm form the digestive tract before the metamerisation. the Mésoderme has a Métamérie. The somatopleure gives the muscles. One distinguishes two muscular unit:

- muscle with fiber circular, external, by contracting reduction in the diameter of the body.

- muscle obliques which joint the dorsal wall with the ventral line médio, flatness of the body.

The splanchnopleure with dimensions interns gives 2 mésentères: the circular apparatus i.e. the dorsal vessel and the ventral vessel. The mésoderme gives the genital apparatus which will form the gonades at metameric disposal. Each coelomic blister emits a channel towards outside. These channels will form the apparatus excretor composed by the néphridie (a pair of néphridie by métamère) which emerges by a pore: the néphridiopore.

Study of a parapode

The parapodes are made of 2 oars:
  • a dorsal → notopode
  • ventral a →neuropode
Each oar is supported by acicula. They are made of foliaceous blades. An extension carries silks articulated (made of 2 parts). Each oar carries a wax (wax dorsal, wax ventral) which has a sensory role. The wax ventral is in contact with the bottom whereas the wax dorsal carries out breathing. The cutaneous respiration is done by wax dorsal.

Morphology and anatomy of the polychètes

The polychètes have a prostomium which is dorsal in front of the mouth. It carries eyes simple, a dental pair and a pair of Palpe (sensory role). Summoned , begin with a structure which surrounds the mouth → péristomium. It carries 2 tentacular pair of waxes = parapode modified. The péristomium results from the fusion of the 2 first métamères. Pygidium , carries the anus and a pair of filaments = the cerques ones (sensory role)

Ecology

To develop…

List orders

According to ITIS:
  • subclass Palpata
  • subclass Scolecida
    • order???
      • family Aeolosomatidae Beddard, 1895
      • family Arenicolidae Johnston, 1835
      • family Capitellidae Grube, 1862
      • family Cossuridae Day, 1963
      • family Maldanidae Malmgren, 1867
      • family Opheliidae Malmgren, 1867
      • family Orbiniidae Hartmann, 1942
      • family Paraonidae Cerruti, 1909
      • family Parergodrilidae Reisinger, 1925
      • family Potamodrilidae Bunke, 1967
      • family Psammodrilidae Swedmark, 1952
      • family Questidae Hartmann, 1966
      • family Scalibregmatidae Malmgren, 1867

External bonds

Simple: Polychaeta

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