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why the coelome is considered as perihaemal system in
phylum echinodermata?
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Answer / vandhana
water vascular system consists of a ring canal, encircling the oesophagus, and five radial canals issuing from it, one for each radius: in many-rayed forms there is a corresponding number of radial canals. From the radial canals pairs of small side branches issue, each ending in a tube foot. At the base of each tube foot there is generally a small vesicle, the ampulla. When this ampulla is compressed, the water (which fills the whole water vascular system) is pressed out into the tube foot which is thus extended. When the ampulla is relaxed and the muscles of the tube foot contract, the water flows back from the tube foot which is thus shortened. A peculiar valvular structure in the ampullae regulates the filling and emptying of the tube-feet. On the ring canal, one or more outgrowths are found, the Polian vesicles. The stone canal connects the ring canal to the madreporite. The water vascular system is one of three parts of the tubular coelomic system. The other parts are the haemal system (hs) and the perihaemal system (phs), which usually surrounds the haemal system.
Haemal and perihaemal system
The haemal system is a system of communicating spaces in a spongy tissue and has probably the additional function of producing coelomocytes (amoeboid floating cells). The perihaemal system is probably solely vascular.
These systems, together with the nervous system (ns), show the same arrangement as is found in the water vascular system: a ring around the oesophagus from which a main branch issues to each radius.
Refer: http://species-identification.org/species.php?species_group=echinodermata&menuentry=inleiding&id=2&tab=foto
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