152 THE EYES OF ARTHROPODS. 



It will be recalled that in arthropods the compound eye is rarely circular in 

 outline. It is usually crescentic or kidney-shaped, the convex margin being turned 

 toward the source of light. A characteristic condition is seen in forms Hke Limulus, 

 trilobites and merostomes, where the eyes are located on the sloping haemal 

 surface of the bucklers, the hght coming from above, when the animal is in its 

 normal crawling position. Here the eyes take the form of convex crescents, or 

 some sHght modification of them, because such a form distributes the maximum 

 number of ommatidia to best advantage in reference to the direction and the in- 

 tensity of light. For similar reasons of economy, the optic nerve reaches the eye 

 at its topographical center, that is, near the middle of its neural or concave margin, 

 and all the fibers are distributed from that point by the shortest paths to their 

 respective terminals. (Fig. 106, .4.) 



Fig. 106. — Diagram to explain the origin of the choroid fissure in the lateral eye of vertebrates. A. The 

 extra cerebral, kidney-shaped compound eye of a marine arachnid; B, the same eye, as is appears seen through 

 the walls of the head in vertebrate embryos. In its transfer to the wall of the cerebral vesicle, the eye is turned 

 upside down and inside out. The arms of the horseshoe-shaped retina then unite, forming a choroid fissure, while 

 the optic nerve, entering near the middle of the retina, distributes its fibers over what is now its outer surface. 



If such a kidney-shaped eye, lying during the early stages near the margins 

 of the cephalic lobes, were actually involved in the brain folds, as the larval ocelli 

 are, it would still tend to retain the same shape and to occupy the same posi- 

 tion that it did while on the outer surface of the body, that is, the eye would 

 eventually grow out from the brain wall, on the end of a tube directed back- 

 ward toward the original position of the eye. 



But the kidney-shaped eye, owing to its inversion during the infolding, would 

 now form a kidney-shaped retina, or sensory placode, with its convex surface 

 directed inward, instead of outward, and its concave margin directed haemally, 

 instead of neurally. (Compare Figs. 32 to 34 and 106.) In other words, 

 the inverted compound eye would have the same peculiar shape and position that 

 the vertebrate retina has at an early embryonic period. But in its new position 

 and under the new conditions prevailing within the head, the open crescentic 

 form of the placode would probably not be retained. It would be Hkely to 

 follow its original method of growth unchecked, till a new position of equilibrium 

 was attained; that is, it would continue to grow more rapidly on one margin than 

 on the other, till the two limbs of the crescent unite, forming a concave, circular 

 retina, with a "choroid fissure" directed haemally, and with a centrally located 

 nerve at the apex of the fissure, distributing its fibers radially over the concave 

 surface of the retina. (Figs. 107 and 108.) 



