Saturday, July 30, 2011

Living fossil in evolution - the Lancelet!

The first animal was taught about to me in the first class of my first Science College I studied. I was not able to get a clear idea about it in the lecture. Earlier I had been taught only about fish, frog and rabbit in Pre-University Course. This animal was a different one - it was neither like a worm or a fish! It is the Amphioxus (Bronchiostoma lanceolatum, Lancelet). It is a primitive animal from which vertebrate animals developed by evolution. It is fish-like and of 2 inches length. It has a transparent body without fins. But has no eyes.

The tentacles hanging down from its mouth are sensory devices that are also used to filter water. Amphioxus takes in water; food particles stick to the mucus inside it and flow down to the digestive tract. Water that entered into its mouth exits out through atriopore via atrium. Though the gills are present, this animal respires only through its skin. There is no heart in its circulatory system or blood cells in it! Sexes are separate. In males, gonads release gametes into the atrium from where they move out with the water through atriopore. In females, eggs come out of atriopore to be fertilised by a single gamete. This animal lies buried inside sands in shallow waters of tropical seas. It is 'harvested' as a food for humans in Asia.
This animal is a wonder animal that is a 'living fossil' in this world as it gives us an evidence of evolution of higher forms of life from lower forms!

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