!64 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



humor being the only refractive medium. The eyes of 

 these are, of course, situated about the head, and often, 

 as in snails, on the ends or at the base of the tentacles 

 or so-called horns (Fig. 102). 



The acephala, or bivalves, as the name indicates, are 

 without distinct head, and the eyes or eye-spots are 

 strung along on the margins of the mantle, as in pecten. 

 But in many lower acephala and in echinodermata and 

 ccelenterata the lens or any kind of refracting image- 

 making instrument disappears, and the eye is reduced to 

 a deposit of pigment to absorb the light and a specialized 

 nerve to respond to light. These are called eye-spots. 

 They differ from true eyes in not forming an image. 

 They perceive light, but not objects. They have the spe- 

 cialized nerve, but not the image-making instrument. 



Arthropods. — We passed over these because out of 

 the direct line of evolution. We now return. 



Many arthropods — for example, the spider — have 

 eyes on the same plan as other invertebrates, but usu- 

 ally very small. But the most characteristic eye of ar- 

 thropods is the compound eye of 

 insects and crustaceans. It is ne- 

 cessary, however, before describing 

 these, to say something of the sim- 

 ple eye of arthropods. 



Simple Eye. — If a spider be ex- 

 amined with a hand lens, a number 

 of brilliant, gemlike spots are seen 

 on the front part of the cephalo- 

 thorax. They are usually in groups 

 of four, six, and eight on each side. 

 Though small, they are somewhat 

 perfect for invertebrate eyes, for we find a cornea, c 

 (Fig. 103), a lens, Z, a vitreous humor, V, a retina, r, 

 and an optic nerve, on. 



Fig. T03. — Eye of a spider : 

 L, lens ; V, vitreous hu- 

 mor ; r, retina ; tr, cor- 

 nea ; on, optic nerve. 



These are the eyes. 



