CHAPTER 9 



EFFECTOR MECHANISMS 



Protoplasm, the agent of organic creation, not only possesses the power 

 of chemical synthesis which we have previously examined, but further- 

 more, exhibits for the purpose of setting such power in play, the faculty 

 of being "irritable," and consequently through its irritability the power 

 of motion. It can, in fact, react by contracting under the action of 

 external excitants. 



Claude Bernard, 1878. Lecons sur les Phenomenes de la Vie, 

 communs aux Animaux et aux Vegetaux. Trans. C. M. Stern. 



The effector structures of the body have two general functions. The first 

 and more obvious is that of carrying out the various actions by which the 

 animal responds to changes in its environment. The other lies in the per- 

 formance of numerous internal acts concerned with furthering the vegeta- 

 tive functions. The structures responsible for these activities are: con- 

 tractile cells and muscles; amoeboid, ciliated and flagellated cells; tricho- 

 cysts and nematocysts ; electric organs ; exocrine glands ; sound-producing 

 organs; chromatophores ; and diverse luminescent structures. 



Movements of the animal are produced by muscles, amoeboid activity 

 or by the beating of cilia and flagella. These structures are also employed 

 in various vegetative functions, in respiration, digestion and excretion. 

 Some effectors are continuously active throughout the life of the animal, 

 e.g. the cilia of many metazoans. Others show discontinuous activity, 

 regulated and controlled by co-ordinating systems. Many effectors are 

 simple structures, consisting of a single kind of tissue — ciliated cell, 

 muscle fibre, etc. Others are complex structures, the functioning of which 

 involves the interaction of several kinds of tissues, e.g. complex chroma- 

 tophores and luminous organs of cephalopods. Chromatophores and 

 luminescent organs are described in following chapters, and glands in 

 conjunction with the organs they subserve. 



CILIA AND FLAGELLA 



These are small vibratile structures located at cell surfaces and performing 

 mechanical work by rapid oscillatory movements. They occur in most 

 groups of animals with the exception of nematodes and typical arthropods. 

 Flagella are elongate threads found on flagellates, choanocytes of sponges, 

 the endoderm of coelenterates and on nephridial flame cells and sperma- 

 tozoa of higher forms. Flagella occur singly or sparsely on each cell. 

 When a cell bears numerous short vibratile organelles these are referred 

 to as cilia, Cilia cover part or all of the external surface of infusorians, 



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