i6 MOTOR ORGANS. 



the animal as well. Cilia usually occur in great numbers 

 and are short. Flagella occur singly, or at most two or 

 three to each cell, and they not only lash the water in a 

 definite direction but often have a spiral motion. 



In the lower animals, such as Protozoa, Porifera and 

 Cxlentcrata, these organs usually act as motor organs or for 

 the purpose of obtaining food. In the higher animals 

 these functions are performed by muscles. 



(2) Muscles. — A muscle is a specially contractile organ 

 which is either in the form of a straight line or a circle. In 

 the former the muscle, upon contracting, reduces the length 

 between the points; in the latter, contraction results in a 

 reduction of the diameter. Nearly all the lower Metazoa, 

 ^sometimes called " worms/' move by a system of circular and 

 longitudinal muscles and their alternate action upon the fluids 

 of the body, as more fully explained later (see Lobworm). 

 Above these, the other Metazoa have the circular principle 

 mainly confined to the sphincter muscles, which close up 

 certain apertures, and to the muscles of the alimentary canal. 

 The great majority of their muscles are of the *'long" or 

 straight-line type, which extend from one fixed point, called 

 the origin, to another attached to the part intended to be 

 moved and called the insertion. These muscles move a 

 definite system of levers and we can observe two great types. 

 In the one form (throughout the Arthropodd) the lever is 

 hollow and contains the muscle, and in the other (in the 

 Vertebratd) the lever is solid and the muscle is placed out- 

 side it. The former conduces to greater actual mechanical 

 advantage, but the latter has infinitely greater possibilities in 

 complexity and nicety of movement. 



3. Sense - Organs. — Sense-organs are parts of an 

 organism in which is specially concentrated the property of 

 irritability. Quite far down in the animal scale, these sense- 

 organs become distinguished among themselves for response 

 to vibrations of a special wave-Jength. It is difficult for us 

 to appreciate any kind of senses other than our own. Our 

 eyes are sensitive to vibrations varying from \76o to X39o^' 



* A = A millionth of a millimetre. The higher wave-length vibra- 

 tions give us the sensation which we call "violet," and the lowest we 

 call "red"; between them lie all the colours of the spectrum. A 

 mixture of all these wave-lengths we term light. 



