Snail how many eyes

Grapevine snail. A snail has something called a radula in its mouth. It is like a file with rows made up of about 25, tiny teeth. Snails produce a colorless, sticky discharge that forms a protective carpet under them as they travel along. This slimy discharge is so effective that they can crawl along the edge of a razor without cutting themselves.

A cup shaped eye develops, when the outer skin epthelium caves in and light sense cells on opposites sides of the resulting cup can tell the difference between light and shadow. The effect is even increased by pigment cells isolating the sense cells against each other and mainly against lateral rays of light. Ocelles in the mantle rim of giant clams have the same construction. How a cup-shaped eye is constructed clearly limits its visual abilities.

Cup shaped eyes can only tell the difference between light and dark and also they can see where the light comes from. But they cannot distinguish forms and they can certainly not see any pictures.

Marine gastropods could only become more mobile, when their abilities of sight had improved. This happened, when the eye cup deepened and the visual opening narrowed. An effect was the result, which in historical time was used by the so-called pinhole camera: A picture of sufficient sharpness can be displayed by reducing the camera's aperture to the size of a pinhole. The smaller the aperture, the sharper the picture. But because there is only a small amount of light falling through the pinhole, the image will be quite dark.

Besides, the displayed area of the image is very limited. A pinhole camera eye today can be found in several marine gastropod groups, such as in ormers Haliotidae and in top shells Trochidae.

Actually, a sea snail's pinhole eye is even slightly improved: The eye's internal cavity is filled by a refractive gelatinous secretion breaking the light rays. As a consequence, the visible image is brighter.

In spite of that, focusing the image is rather difficult. So it may be assumed that ormers and top shells more like see forms and shadows than a real picture. This eye type may be sufficient for slow herbivorous snail species.

But a precondition for the evolution of faster carnivorous species was a better visual organ, which enables the snail to see pictures and to recognize them.

In the course of this evolution, the outer skin epithelium grew over the eye's opening the pupil and a transparent layer of cells closed the eye to the outside. The refractive secretion in the inner eye developed into a round bubble or vesicle with better refractive properties.

This type of eye is called a vesicular eye. Vesicular eyes are found in many gastropod groups in the sea and in fresh water, such as the apple snail Ampullaria. Vesicular eyes have much better visual properties than more primitive eye types. Especially predatory gastropods which is a large portion of the sea-living species now could better recognize and follow their prey.

A friend left him on our doorstep when they knew we were looking for a photogenic model. After his first photo shoot Spot escaped from a temporary bucket by pushing up a weighted screen — slugs can be amazingly strong. He was on the loose for several days in the sluggerium and then suddenly for a slug turned up inside our large holding pen.

Since this has a solid glass top the only way he could have entered was by burrowing under the wall and breaking into jail! Spot is now living the good life with plenty of company, fresh lettuce, carrots and catfood every day. Retracted tentacles. First to come out are the lower tentacles. Next come the eyestalks. The eyestalks push out from the center. Like an inside-out sock. All Rights Reserved.