PHYSIOLOGY OF CELLS 



65 



Myonemes. — In the stalk of Vorticella, one of the Protozoa, there is 

 a speciahzed portion of protoplasm the function of which is contraction. 

 Here the protoplasm occurs in fine thread-like strands, called mijonemes, 

 which are probably not very different from the contractile elements in 

 cilia and flagella. The myonemes may be considered to be the fore- 

 runners structurally of muscular fibrils and a more complete discussion 

 of them will be deferred to the discussion of muscular contraction in 

 Chapter VII. 



mes 



Fig. 32. — Portion of cross-section of the sponge Grantia. cc, collared cells of endoderm. 

 ect, ectoderm; /?, fiagellum of collared cell; mes, mesoglcea; sp, spicule (portion only); 

 {Original.) 



Irritability. — A further distinction ])etween living- and lifeless things 

 is that living things respond to stimuli. Stimuli are changes of the en- 

 vironment or the protoplasm itself of such an intensity as to cause an 

 organism to make a change in its position, direction of movement, shape 

 or color, or give some other form of reaction. Some kinds of external 

 stimuli are change in the intensity or direction of light, change in tem- 

 perature, pressure, gravity, or contact, and applications of an electric 

 current, solutions of acids, salts, or bases. Hunger, thirst and fatigue 

 serve as examples of internal stimuli. This property of living things 

 which causes them to make responses to stimuli is called irritability. 

 In the lowest forms of animals (the Protozoa) there are no specialized 

 structures for the perception of stimuli or the conduction of impulses. 

 Such functions must be performed b}^ the general protoplasm. The 



