File:Animal biology (1940) (17574286544).jpg

Title: Animal biology
Identifier: animalbiology1940wolc (find matches)
Year: 1940 (1940s)
Authors: Wolcott, Robert Henry, 1868-1934; University of Nebraska (Lincoln campus). Dept. of Zoology
Subjects: Zoology; Biology
Publisher: New York ; London : McGraw-Hill Book Company, Inc.
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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260 METAZOAN PHYLA
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Ommaticiium Corneoi ■Basement membrane 294. Eyes and Vision.—The crayfish possesses com-pound eyes. Each eye is hemispherical in form and is covered by a transparent cornea which represents a modified portion of the cuticula (Fig. 166). The cornea is divided into rectangular facets, each one of which is the outer end of a rodlike unit known as an ommatidium. These ommatidia —of which there are approximately 2500—are radially arranged rods tapering toward the base, which causes their axes to converge toward a common center. From the ommatidia lead nerve fibers which, together, make up the optic nerve. When an ommatidium is observed carefully it is seen to consist of a corneal facet at the outer end (Fig. 167) beneath which are corneagen cells which secreted it. Inward from these are four elongated cells, mtrellae, which form a crystalline cone near their distal ends. The distal and proximal ends of the vitrellae are enveloped by several retinular cells, sensory in function, from which nerve fibers extend inward to optic gangha. The rhabdom, a refractive body secreted by the proxi- mal retinular cells, lies in the axis of the ommatidium at its inner end. The cor- neal facet, vitrellae, crystalline cone, and rhabdom are all transparent. The re- tinular cells cover the surface of the ommatidium and contain pigment. In bright hght this pigment is distributed throughout the length of the omma- tidium; in dim light, however, it con- tracts toward the respective ends leaving much of the surface without this dark covering. A compound eye sees just as many Httle images as there are omma- tidia, and since these images together make up the whole of the picture received by the animal, the picture has been termed a mosaic image. There is, however, some overlapping of the separate images. The pro- duction of a separate image for each ommatidium results from the fact that each of the ommatidia is long and slender, and since the pigment along the sides absorbs all the rays which it receives, only those rays reach the bottom and stimulate the retinula which are practically in fine with the axis of the ommatidium. In dim hght the withdrawal of pig- ment from the wall of the ommatidium permits all of the rays entering it to be reflected inward, increasing the amount of hght falhng upon the retinula and thus giving a stronger stimulation. This probably does not result in a clear image but enables the animal to distinguish between light and dark. A compound eye has a great disadvantage when compared with such an eye as the vertebrate eye in that the animal cannot focus with it, thus hmiting the distance to which vision is possible. It has, however, the great advantage that it more readily perceives move- FlG. Opf/c nerve 166.—Diagrammatic representa- tion of the eye of a crayfish.

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