All branches of the plant world and all branches of the animal world 

 are sensitive to light. But only three main groups of animals can actually 

 see. These are the highest mollusks, the arthropods, and the vertebrates. 



By seeing we mean not simply discriminating between light and dark, 

 but distinguishing forms at some distance. The starfish, for example, has 

 light-sensitive spots at the ends of the rays, but these are not true eyes (see 

 illustration, p. 230). Comparatively few of the mollusks have special light 

 organs. In most of the bivalves the edge of the mantle is vaguely sensitive 

 to light. The scallops have "eyespots" at the edge of the mantle, but in the 

 snails, the squids, and the octopuses there are definite eyes. The eye of the 

 octopus resembles that of the backboned animals in many ways. 



Vertebrate Eyes Among all backboned animals the eyes are very 

 much alike (see illustration opposite). Important differences correspond to 

 the habits and the habitats of the different groups. Animals living in the 

 water, for example, have a different kind of lens. Animals that prowl about 

 at night have a different kind of pupil. The eye is moved about in its set- 

 ting by muscles attached to the bony framework, and is further protected 

 by the movable lids and watery secretions. 



The fishes (except the sharks) lack eyelids. The eyelids of snakes are 

 permanently closed, but transparent. Among the birds and in many reptiles 

 there is a single eyelid that passes over the eyeball from the inner corner, 

 under the outer pair of eyelids. 



Compound Eyes Insects and other arthropods commonly have com- 

 pound eyes, and many of them have also simple eyes. In each of the eyes 

 there are many nerve-cell endings. The lens projects upon these sensitive 

 points a tiny patchwork of varying lights and shadows. Thus each of the 

 many eyes forms some tiny picture of a portion of the outside world. 



A compound eye of an insect or lobster may have from twenty to several 

 thousand separate facets. The impressions produced in the units of a com- 

 pound eye are probably not very distinct. But as the animal gets a mosaic 

 of many simultaneous views from somewhat different angles, it is disturbed 

 by very slight movements. Most insects are able to detect movements in 

 practically all directions, though not at a very great distance. 



The Senses and Adjustment^ Most of the organs through which we 

 receive stimuli from the outer world depend upon direct contact between 

 the body and some object. Reaction to such stimuli is ordinarily immediate 

 — of a reflex character. If an animal is to profit from its ability to sense 

 such stimuli, it must respond promptly. And if the stimulus comes from 

 possible food, the reaction must take place before the food has time to 

 get away. 



The three senses that enable an organism to receive stimuli from objects 



^See No. 9, p. 300. 

 290 



