882 



COMPARATIVE. HISTORICAL. 



may consist of single crystalline cones, surrounded by pigment (nervous end- 

 organ), are found in the neighborhood of the brain (the larvae of some crabs), 

 or there may be several crystalline rods in the compound eye (lower crabs). 2. 

 Eyes with cornea, which consists of a lenticular shaped, chitinous formation of 

 the outer external integument, are found to be either simple, consisting of a single 

 crystalline rod, or compound. The latter have either only one large lens-shaped 

 cornea, common to all the crystalline rods, as in spiders (Fig. 310); or each 

 crystalline rod possesses, for itself a special lens-shaped cornea. The numerous 

 rods, surrounded by pigment, are placed close together, and form a curved sur- 

 face. The chitinous covering of the head is faceted, and forms a cornea-lens 

 on the surface of each rod (Fig. 311). There are two theories as to the way in 

 which the image is produced by this compound eye of the arthropods. According 



rz 



k 



FIG. 310. Eye of the Cross-spider, according to 

 Grenacher; decolorized: cl, cornea-lens; 

 hz, hypodermal cells; b, basal membrane; 

 gkz, vitreous cells; rz, retinal cells; k, nuclei 

 of the retinal cells; s, rods; n, nerve. 



to one, each facet, with the lens and crystal sphere, 

 is a separate eye: while man has two eyes, the in- 

 sect is supposed to have many hundreds of eyes. 

 Each eye sees the image of the outer world as a 

 whole. The following experiment of Ant. Leeuwen- 

 hoeck seems to indicate this: If the cornea is cut 

 off, each of its facets forms a separate image of 

 objects. If, for example, a cross is placed on the 

 mirror of a microscope, while a piece of faceted 

 cornea is plated as an object on the stage, an image 

 of the cross can be seen in each facet. Consequently 

 a separate image would be formed for each rod 

 (crystal-sphere). This takes place, however, only 

 when the crystal-sphere is removed. In combina- 

 tion with the latter, each corneal facet forms only 

 a part of the (upright) image of the external 

 world, so that the image must be conceived to be 

 composed like a mosaic (mosaic vision). The 

 Rontgen rays appear to be visible to insects (flics) . 

 Among molluscs the fixed brachiopods have two 

 pigment-spots near the brain, but only in their free 

 larval condition. Similarly, the larvae of mussels 



have pigment-spots with refractive bodies. Adult mussels, however, have pigment- 

 spots only at the margin of the mantle, but some of them have pedunculated, 

 emerald-lustrous highly developed eyes. Among the snails several of the lower 

 forms possess no eyes at all, others have a pair of pigment-spots on the head, and 

 a number have eyes in various stages of development (Figs. 312, 313). The 

 garden-snail has its eyes on a special pedicle, and they are provided with a cornea, 

 optic nerve, retina, and finally, even lens and vitreous. Of the ccphalopods 

 the nautilus has no cornea or lens, and the seawater flows freely into the ocular 

 cavity. Others possess a lens, but the cornea is absent, while still others have an 



FIG .311 . Individual Eye of a Libel- 

 lula Larva (Dragon-fly), diagram- 

 matic and simplified, according to 

 Carriere: a, longitudinal section; 

 b, cross-section; cl, cornea-lens; 

 kz, crystal sphere (cells); hz, hy- 

 podermis cells; p, pigment-cells; 

 rz, retinal cells, surrounding rh , 

 the retinal rod. 



