THE ANIMAL ACTION-SYSTEMS 73 



of the lash. The ciHum then recovers, or extends more slowly. 

 Even in the small, microscopic protozoa there are great numbers 

 of cilia and these are suitably arranged and lash in co-ordination. 

 Extraordinary variety of movements, involving most complex 

 co-ordinations of the cilia, occur and the complexity of this mode 

 of movement cannot be much less simple (if at all) than the move- 

 ments of the body and limbs of a mammal. 



Ciliary movement is common in almost all groups of animals, 

 in respiratory, cleansing and other movements. Thus the water 

 currents into and out from the mantle cavity of such an animal 

 as the mussel, or oyster, is maintained by cilia and even in man 

 ciliary motions occur in the trachea, removing mucus, etc., into the 

 mouth. In many animals the movements of food matter through 

 the alimentary canal are maintained by cilia. 



24^. Flagellate Movements. Each cell in the sponge has 

 a motile, hair-like organ, a flagellum, or large cilium. The lashing 

 of these establishes the incurrent and excurrent water movements. 

 Hosts of protozoa and algal spores swim by the lashing of one 

 or more flagella. 



The writhing motions of the bodies of spirochaetes, or the 

 motions of spermatozoa are analogous. The whole spirochaete, 

 or the tail of the spermatozoon writhes (like the tail of a tadpole 

 or the whole body of an eel). 



Many bacteria (minute as they are) are provided with groups 

 of flagella and move by the lashing of these organs. 



These are examples of action-mechanisms. In each there is 

 a complex arrangement of motile parts — limbs, appendages, 

 tendons, bones, muscles — all controlled by peripheral and central 

 nervous organs and sensory parts or systems of suckers, tube-feet, 

 etc., also under sensory and nervous control, or systems of cilia 

 which do not have any nervous parts associated with them. In 

 all cases every such mechanism has a general structural plan and 

 it is capable of a certain variety of modes and amplitudes of move- 

 ments. These modes of movement can be varied, by the method 

 of trial and error, and by the experience of the animal, but the 

 possible varieties and amplitudes are, of course, limited by the 

 structural plans that have been evolved. 



So far as we can see, all action-systems are actuated by muscles, 

 or by ciliary and flagellate parts, which apply forces in some way 



