52 



PRINCIPLES OF ANIMAL BIOLOGY 



Many of the simple unicellular animals and some of the multi- 

 cellular ones perform movements by means of cilia or flagella. The 

 cilia are minute hairlike projections capable of rapid vibration. 

 Each cilium has an elastic outer layer containing one or more con- 

 tractile threads within it, as in Fig. 31. Contraction of the threads 

 on one side bends the cilium in that direction, and elasticity of the sheath 

 causes it to return. 



In the vigorous stroke of a cilium, it is extended and moderateh' 

 stiff, so as to catch much liquid; on the return stroke it bends limply 

 nearer the surface of the cell (Fig. 32). Neighboring cilia usually beat 

 in unison or in waves. 



Flagella differ from cilia chiefly in their greater length and are few 

 in number (usually one to eight per cell). Sometimes the flagellum is 



surrounded by a vaselike collar (Fig. 33). 

 Flagella may beat regularly in one plane, as do 

 cilia, or they may have a rotary motion. The 

 whole flagellum may move, or only the free end of 

 it. The flagellum is composed of an elastic per- 

 ipheral layer within which are several contractile 

 threads (Fig. 34), and the movement is due to 

 '^^s the contraction of these threads. Flagella give 

 a motile cell a jerky erratic movement; cilia cause 

 it to glide. 



Fig. 33.— Portion of ReSDOnses to Stimuli. — A characteristic pro- 



cross section of the sponge ,• 1- • J, ■ -i 1 •!•, J 1 i 



Grantia. cc, collared cells perty ol livmg matter is its ability to respond to 

 of endodenn; ect, ecto- stimuli. A stimulus is anv influence of sufficient 



derm; fl, flagellum of col- • i , \ • , i 



lared cell; mes, mesogioea; magnitude to cause a change m protoplasm. 



sp, spicule (portion only), ^j-^g stimulating agent may be external to the 

 organisms, such as changes in light, temperature, chemical substances, 

 sound, pressure, or electric current; or it may originate within, through 

 osmosis, electric charges, chemical substances, pressure, or nerve impulses. 

 To be a stimulus, the modification must have a certain degree of 

 suddenness. A very gradual change in the intensity of light may have 

 no observable effect, while a sudden change of the same amount produces 

 a marked reaction. 



Responses are of very different sorts. Muscle cells and others con- 

 tract; gland cells produce secretions. Pigment cells in the skin of a 

 frog, which are highly branched and have their pigment distributed 

 throughout all parts of the cell when at rest, contract their pigment into 

 a small compact mass in response to light, thereby changing the animal's 

 color. Streaming of protoplasm in plant cells stops in response to an 

 electric current. A chemical substance in the retina of the eye of 



