Pentamerism
Pentamery is the biological technical term for five-pointed radial symmetry , a unique phenomenon in the animal kingdom. It is a common property of echinoderms , but is known as secondary pentamerism. The larvae of the Echinodermaten are morphologically considered bilateral - symmetrical and develop a pronounced radial symmetry only later in life and as a result of a metamorphosis . This can be seen particularly well in the Asteroidea (starfish); This morphological expression, which is unique in the animal kingdom, is also found in the other classes of echinoderma.
The five-rayed structure arises - genetically controlled in the direction of five radii - when the (external) skeleton , the stereome, grows - through the effect of its own cell associations (sclerocytes). The resulting structure is highly porous and network-like - a construction that is both light and stable - as can be seen from the remains of a sea urchin . The five star-shaped growth radii contain a system of different vessels (coelom spaces) and nerve tracts.
The five-pointed radial symmetry can occur in many Pelmatozoa and also in some starfish, so that z. B. the crown of thorns starfish can have about 20 arms. This superimposed symmetry only develops in the course of individual development ( ontogenesis ), whereby the animal initially only exists as a bilaterally symmetrical larva .
For special symmetry of the echinoderms of the specific tissue type is still Mutablen Bindewegewebes (MCT, mutable connective tissue ). It allows the outer shell of the Echinodermata to change its stiffness without great expenditure of energy and muscle movement . B. to achieve special filter positions . Many echinoderms can also re-create body parts that have been bitten off. The spines themselves are hollow forms of the skeleton made of calcium carbonate (CaCO 3 ) that have grown outwards , which regenerate particularly quickly if they break off and can also be locked in place via the mutable connective tissue.