Posts Tagged ‘light-sensitive cells’

Eyes Of Invertebrates And Vertebrates

Saturday, December 19th, 2009

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.

Vision And Optical Analysers

Wednesday, December 16th, 2009

The optical analyser developed under the influence of sunlight, so it did not need to be so complicated. The eyes of various animals can perceive no more than a three-octave light flux. The range of light perception is thus only one-fifth that of the sound range. Few animals on our planet are indifferent to light. Even the eyeless protozoa can distinguish light from darkness. Sensitivity to light is based on the property of some chemical reactions to be accelerated in the presence of light. Hence, the protoplasm of practically any cell in a multicellular organism can perceive light, and it needs no eyes for this purpose.

The forerunners of the organ of vision were special light-sensitive cells which could react to feebler light than the other cells in the organism. There are some creatures in which these special light-sensitive cells still exist. One that we know well is the earthworm. It has no eyes, but is quite happy with the numerous light-sensitive cells in its skin. With these cells it feels negligible changes in illumination, which man ca’nnot perceive. It was from such light-sensitive cells scattered all over the body that the eye gradually developed in the course of evolution. At first it was just an accumulation of light-sensitive cells in one spot. Such eyes readily distinguish light from darkness, but they cannot tell from where the light is coming.

The eye then evolved in the following way. The light-sensitive cells gradually acquired a transparent cover and screens of pigment cells that did not allow light into the eye from all directions. Then the light-sensitive spots turned into pits, or even sacs, the first eyes worthy of that name. These eyes could catch light coming only from a certain direction and easily established thereby the direction of the incident light rays. From this primitive optical device there remained but one step to the eye of higher animals: the eye only had to acquire refractive and accommodation systems modifying the refractive index and, last but not least, the oculomotor apparatus that made an active search for visual information possible.