Cephalochordata

The Cephalochordata ( Céphalochordés ) or Acrâniens, represented by Amphioxus ( Branchiostoma lanceolatum ), are marine animals of relatively rudimentary organization rather similar of appearance to the Poisson S. Including/understanding approximately 25 species, they live in dark water of the moderate and tropical oceans.

Other orthographies: amphoixus, ampihoxus, amphioxys, amphioxis, amphiocus, amphiixus…

Characters

General characters resulting Chordés

Their body is long and compressed laterally. They have the five characteristics defining Chordés:

Presence of a notochorde (cylindrical structure mesodermic dorsal lengthened which plays a part of support).
dorsal nervous Tube, compared to the intestine and with the chorde in the shape of hollow tube (Epineurien).
Slits pharyngiennes (Pharyngotrème).
Endostyle (fold which recovers the face ventral of the pharynx and which conveys the food particles with the digestive tract by secreting a mucus).
Myomères (striated muscles).
station-anal Tail.

Clean characters

Absence of differentiated head (definition of the Acrâniens ).
Chorde presents over the entire length of the animal (definition of the céphalochordés ), ensuring rigidity (support) longitudinal axis.
longitudinal Muscles laid out out of rafters (myomères), separated by conjunctive fabric (the muscles represent about half of volume of the animal). The alternate contraction of the myomères on each side of the animal causes undulations which enable him to swim or to hide in the sediments.
ciliées Slits pharyngiennes: creation of a water current penetrating by the mouth then rejected by the atriopore. The filtered particles stick to mucus slits pharyngiennes and are forwarded to the digestive tract.

The subphylum of céphalocordés is characterized by the former prolongation of the notochorde, without development of a brain, and by little developed sensory capacities as a whole, compared to other Chordés. Long from 6 to 8 cm, alive in sea-beds with depths between 1 and 25 meters, they nourish organic particles present in the sediments which they filter between the mouth and the slits branchiales.

Céphalochordé the most widespread is the amphioxus ( lancelet in English) which measures a few centimetres length and lives in the underwater sandy zones. Oldest céphalochordé known from the lower Cambrien dates.

Classification

According to ITIS:

order Amphioxiformes

family Branchiostomatidae

kind Branchiostoma Costa, 1834
kind Epigonichthys Peters, 1876

According to others:

order Amphioxiformes

family Asymmetronidae
family Branchiostomatidae

Phylogeny

Two alive kinds of Céphalochordés were described: the kinds Branchiostoma and Asymmetron, representing on the whole 25 species.

Interest of study

Very studied in animal Biology because their absence of pigmentation makes it possible to observe by transparency all the internal bodies without having to dissect them, the amphioxus became a model of Study in Evo-Devo to include/understand the mechanisms of evolution acting in the transition invertebrate-vertebrate.

Anatomy

Céphalochordés are microphages. The Eau penetrates in the oral cavity then in the cavity pharyngienne then crosses the 90 slits pharyngiennes. The suspended particles are then recovered by a Mucus produced by endostyle, a fabric made up of glandular cells and cells ciliaires (and which is able to metabolize the Iode: it is probably the counterpart of the gland Thyroïde of vertebrate). The slits branchiales are supported by branchiaux arcs. The Sang circulates there in aortic arcs where the Hématose occurs thus.

Urochordata

Sources

Chen, J. there., Dzik, J., Edgecombe, G.D., Ramsköld, L., and Zhou, G. - Q. 1995. has possible Early Cambrian chordate . Nature 377:720 - 722.
Ruppert, E.E and Barnes, R.D. 1994. Invertebrate Zoology . Sixth Edition. Saunders Publishing College, Fort Worth.
Introduction to the Cephalochordata : http://www.ucmp.berkeley.edu/chordata/cephalo.html
Hickman, Cleveland P. and Al 2006. Integrated Principles off Zoology . Thirteen Edition. McGraw Hill, Higher Education. p.482-485 and 488-489.

External bonds

Embryonic development Amphioxus development

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