quinta-feira, fevereiro 04, 2010

The evolution of the digestive system in a phylogenetic context for non-vertebrate deuterostomes

The “digestive” system, if we can call so, in hemichordate establishes the basic pattern that will be subsequently acquired by cephalochordates, urochordates and craniates. The tripartite composition of hemichordates (trunk, proboscis and collar) all interfere in the digestive system. Enteropneusts have a linear body where the proboscis either serves as a burrowing device but also to conduct food items to an aperture in the collar where water comes in, exceeding water is secreted away from the pharyngeal slits (paired in early ontogenesis and multiple in advanced ontogenesis), the food items pass through a duct and will eventually be secreted in anus at the posterior tip of the acorn worm. In pterobranchs the proboscis is composed of several cirrus which conduct food items to an aperture in the collar, in this animals the U-shaped digestive tract implies that the anus will be near the mouth.
In more derived pharyngotremates, such as the tunicates and cephalochordates an important feature called the endostyle further increases the food gathering efficiency. Now not only the pharyngeal slits are capable of sieving food items but also the mucous produced by the endostyle serves that function. In cephalochordates (e.g. amphioxus) the pathway of the food items is: enter the mouth by a complex network of oral cirri which conducte by the help of the wheel structure transport the food to the esophagous which is devided in the epipharingeal duct, further posteriorly the ileocolon ring may store nutrients in the cecum or send exceeding remains through the anus. Exceeding water also passes through the pharyngeal slits which then reach an atrium and are then expulsed through a more anterior aperture relative to the anus.
Echinoderms have a very simple digestive system also formed by a duct but complemented by the ambulacral system that also serves for gas exchange and even locomotion. There are no gills or slits in echinoderms.


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