60 HUMAN PHYSIOLOGY 



lowlier type of organism, the sea anemone. This 

 animal is found adhering to the bottom of pools of salt 

 water along rocky coasts. Its appearance is that of a 

 vegetable rather than an animal type; it is a short 

 cylinder crowned with a circle of tentacles. Inside the 

 ring of tentacles there is a flat surface surrounding a 

 mouth. The zone between the tentacles and the mouth 

 is ciliated. If a grain of sand is dropped upon this 

 area it is seen to move away from the oral opening. 

 Thus the cilia are ordinarily so acting as to protect the 

 animal against the tendency to become filled with use- 

 less debris. This is the more important because the 

 simple digestive cavity of the sea anemone has no 

 outlet. 



If the experimenter places a morsel of meat instead of a 

 sand grain upon the surface which he is observing the 

 result is very curious. The bit of meat at first moves a 

 little away from the mouth. Then it comes to a stand- 

 still and in a moment more it is journeying toward the 

 mouth into which it presently falls. The chemical 

 composition of the edible particle has been such as to 

 modify the action of the ciliated cells. The mechanism 

 of this reversal might be pictured in various ways, but 

 such attempts would be purely speculative and we 

 will content ourselves with the mere statement of the 

 facts. 



Muscular Tissue. Ameboid cells are found singly and 

 the activities of one are independent of those of another. 

 Ciliated cells, as seen in the higher forms, are in a fixed 

 association. The evident movements of such forms 

 are produced by the coordinated working of cells organ- 

 ized into those tissues which we speak of as muscular. 

 Mammalian muscle is of three principal varieties. The 

 kind which seems distinctly the most primitive is 

 that which we call plain or smooth muscular tissue. 

 This is found chiefly in the internal organs and it is 

 sometimes termed visceral muscle because of this fact. 

 A unique sort of muscle is found in the walls of the heart 



