310 ANIMAL LIFE 



mollusks have no eyes, but they are usually sensitive to light. 

 The simplest eyes are found in the jellyfish, and organs of 

 sight somewhat resembling those of the vertebrates are 

 present in the scallop (Pecten) and cephalopods (squid, cut- 

 tlefish, etc.). The snail has well-developed eyes on stalks. 

 The insects and crustaceans have acute vision, but at 

 a distance of only a few feet. Both compound and simple 

 eyes are present in most insects. If the compound eye of 

 a house fly be placed under the low 

 power of the microscope the surface 

 will appear marked off in hexagonal 

 areas. These are the ends of cones, 

 each of which may function as a sepa- 

 rate eye. The queen bee has five thou- 

 sand of them, the drone twelve thou- 

 sand > the dra S n fl y twent y thousand, 



tenna ; e, compound an d SO me beetles twenty-five thousand. 



eye; the three black . 



dots are the simple Ihe simple eyes, called ocelli, located 

 near the middle of the forehead, are 

 just visible to the naked eye in such large forms as the 

 locust in which there are three. These are probably used 

 for very near vision. Spiders have no compound eyes, 

 but from six to eight ocelli larger than those of insects. 



22. PROTECTION FROM ENEMIES 



Every living animal from the amoeba to man has enemies 

 seeking its destruction, and it is only by special adapta- 

 tions that the species persists. The huge elephantlike 

 Titanotheres and Uintatheres which a few millions of 

 years ago roamed about in Western United States, became 



