16 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 
Cwlenterata, 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 Afetazoa, 
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 AZe¢azoa 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 ovigzn, to another attached to the part intended to be 
moved and called the zusertion. These muscles move a 
definite system of levers and we can observe two great types. 
In the one form (throughout the Arthropoda) the lever is 
hollow and contains the muscle, and in the other (in the 
Vertebrata) 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-length. It is difficult for us 
to appreciate any kind of senses other than our own. Our 
eyes are sensitive to vibrations varying from 760 to A390* 
*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. 
