Red-eye flight - Wikipedia
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Foundation gives trip of a lifetime for Easton mom battling stage 4 cancer 3 days ago. Especially the longer tentacles with the eyes are often withdrawn. This poses the question, how good a snail's eyesight really is.
Among gastropods, terrestrial pulmonate snails, such as the Roman snail, are among the most highly developed groups. This might best be explained by those snail groups having had to undergo most evolutionary changes to adapt to life on dry land. Comparing the eyes of all gastropods, all possible evolutionary stages can be seen. Like among all molluscs, in snails as well the development of the sense of sight is closely connected with their way of life. But, contrary for example to bivalves - mussels and clams - snails always have a head, even if they live in an almost immobile fashion.
Especially in those gastropod groups with an almost immobile way of life, the simplest construction of eyes can be found. Among those snail groups mainly are the limpets Patellidae.
The Eyes of Snails
Limpets never leave the rock they are at home on, and their movement is limited to the path between settling place and feeding place. For their needs, therefore, a simple cup shaped eye is quite enough. 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.
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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. But a very large part of hunting in gastropods still is following the prey's scent. From the vesicular eye the lens eye developed when the vesicle of refractive secretion became more consistent, further improving the quality of the picture seen.
Real lens eyes, however, are mainly found in cephalopods.