32 AN INTRODUCTION TO ZOOLOGY. 



of the epidermis, and all contain keratin. Similarly 

 tlie hairs of invertebrate animals, such as the lobster, 

 are strengthened by cbitin. The skin has not only to 

 protect the body, but it has also to feel for the body; 

 it is the organ of touch, and in the highest animals is 

 lull of nerves. The hairs which apppar on the skin, 

 though usually merely a protection, are sometimes 

 organs of sensation of some kind, with little nerves at 

 the base. Thus, the cat's fur is protective, and her 

 whiskers sensitive. We speak familiarly of the five 

 senses, sight, hearing, taste, smell, and touch : the 

 four latter senses are conveyed through cells which 

 are specially modified skin cells, with their appen- 

 dages. In the invertebrata we find comparatively 

 rudimentary sense-organs, consisting of cells with 

 liair-like processes. A cell with one of these pro- 

 cesses is the primary and simplest form of sense- 

 organ. The ear is formed by modification and 

 multiplication of such cells in connection with some 

 arrangement for magnifying the force of sounds. 

 The simplest form of ear is an otocyst, or cavity, 

 containing a small solid body, called an otolith or 

 ear-stone, floating in a surrounding fluid. In fif. 9, 

 p. 31, are shown the otocysts of Mysis, the Opossum 

 Shrimp, which are situated, strange to say, in its tail. 

 The long hairs which fringe the flaps of the tail 

 doubtless assist the perception of vibrations. 



The sense of taste, in ourselves, is accomplished 

 through the means of delicate elongated cells aggre- 

 gated into a round mass; these are called taste- 



