PHYSIOLOGY OF ORGANS 



155 



the members of the opposite sex, and the disagreeable odors of certain 

 insects and mammals by means of which they repel other animals. Un- 

 common instances of glandular activity are the production by glands in 



B 



Fig, 114. — Euglena. A, individual showing flagellum (fl); B, fibrils in flagellum. 

 original; B after Dellinger in Journal of Morphology.) 



iA 



certain fish of an electric current sufficient to cause a decided shock, and, 

 in other animals, of secretions which by oxidation produce light. 



Muscular Contraction. — Protoplasm is contractile, but vigorous con- 

 tractions are possible only when the proto- 

 plasm has developed within it specialized 

 contractile structures. These are of a 

 fibrillar nature. In Protozoa possessing 

 structures capable of strong contraction 

 these fibrils are known as myonemes. 

 Myonemes may be found in the flagellum 

 of such animals as Euglena and Peranema 

 and in the stalk of Vorticella, as mentioned 

 in Chapter III. The flagellum of Euglena 

 (Fig. 114) or Peranema is constructed of 

 an external layer of elastic substance en- 

 closing a number of myonemes which ex- 

 tend from the tip of the flagellum into the 

 cytoplasm of the cell. The myonemes are 

 twisted in the form of a loose spiral. Con- 

 traction of the myonemes or of certain of 

 them bends the flagellum. The stalk of 

 Vorticella is likewise composed of a sheath 



of elastic material enclosing a core made of myonemes (Fig. 115), Con- 

 traction of the myonemes causes the stalk to assume the form of a closely 

 wound spiral. The spiral form is due to the fact that the myonemes 



A B 



Fig. 115.— The stalk of Vor- 

 ticella. A, arrangement of myo- 

 nemes -within the elastic sheath; D, 

 stalk contracted. 



