Sense Organs Evolution

December 19, 2009

Eyes Of Invertebrates And Vertebrates

Filed under: organ of vision — Tags: , , , , , , , — sense organ @ 8:59 am

Cephalopoda possess the best eyes among invertebrates. Their vision is in no way inferior to that of higher ver­tebrates. Another branch of invertebrates, the arthropods, attained a high level of evolution, but, for some reason, this did not apply to their eyes. They compensated for this shortcoming by combining a great number of ocelli (pyramids with the base turned outwards and covered by a chitinous crystalline lens) into a few big eyes of an involved pattern, each eye consisting of hundreds and even thousands of such pyramids. By the joint efforts of their individual, usually rather short-sighted ocelli, insects and Crustacea can perceive the size and shape of objects.

The history of the eyes of vertebrates began in a diller-ent way. The otl-shore area of many seas and oceans is inhabited by curious small animals called lancelets. They look like small fishes and resemble the blade of a surgeon’s lancet, from whence their name came. The organ of vision of the lancelet is its brain. Light-sensitive cells are scattered all along the nerve trunk of the lancelet which has a trans­parent body. It can thus differentiate between light and darkness, which is all it needs for its way of life.

Apparently, the ancestors of the vertebrates, like lancelets, also saw with their brain. But when their bodies had lost their transparency, bundles of light-sensitive nerve cells had to move otV the brain outside. This has become the pattern of the evolution of the eyes in all vertebrates. At a certain phase in an embryo’s development two pieces separate from the brain and gradually develop into eyes. So, our eyes are, in fact, pieces of the brain that have moved outside to the sunlight.

The further development of the eyes in vertebrates fol­lowed the same pattern: they acquired refractive and accom­modation systems and muscles which move the eye. The design became more and more involved until it resulted in our present eyes, capable of deciphering the jungles of the worst scrawl in the world and of distinguishing the slightest tints in colour. At the same time the animal brain also became more complex. The eye as such is merely a light-receiving device, like a camera. What we actually ‘see’ with is the brain. The brain pieces together the information it receives from the millions of the light-sensitive cells in the eye into a single picture. The snapshots made with the eye are developed in the laboratory of the brain.

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