THE BODY TEMPLE 65 



from the eye-spots back into the creature's little brain, 

 where they unite with certain cells which have charge 

 of the sense of sight. 



The leech has the simplest kind of a seeing appa- 

 ratus. The human eye is made upon essentially the 

 same plan. The eyeball, as the round globe of the eye 

 is called, has a dark colored lining at its back part, 

 and a transparent membrane in front. 



Just inside of the delicate lining is stretched out 

 the membrane which contains the nerves of sight. The 

 space between this and the transparent membrane in 

 front is filled up with transparent substances through 

 which the light readily passes. In front, and just be- 

 hind the transparent portion, is placed a little curtain 

 with a hole in the center, which contracts and enlarges 

 according to the intensity of the light, so as to protect 

 the delicate nerves of sight from injury upon exposure 

 to light which is too bright. 



A snail's eyes are perched up on two long stems 

 which most people call feelers. "When he wants to look 

 in different directions, he simply twists his eye stalks 

 around; and when he is frightened, he pulls them in 

 and tucks them away in two little pockets in his head. 

 Our eyes are set back in deep, bony sockets so as to be 

 protected all the time, and we are less troubled caring 

 for them. 



Our Ears.— The little barnacle which passes its life 

 clinging to the slimy bottom of some old ship, has the 

 simplest kind of an ear imaginable. The organ in this 

 animal is a little sac filled with fluid, which hangs in 

 a chamber communicating with the surface, and filled 

 with sea water. In the walls of this sac are distributed 

 the nerves of hearing, for which, in its vegetable-like 

 life, it can have very little use. 



