LECTURE XX. 

 IRRITABILITY (continued]. 



II. The Irritability of Mature Organs. 



HITHERTO we have confined our attention to the manifes- 

 tation of irritability by growing organs, but now we turn to 

 the study of the phenomena of irritability presented by organs 

 which are not growing. 



The various movements which we have to consider under 

 this head will be taken in the following order: first, movements 

 which involve the locomotion of entire organisms : secondly, 

 the streaming movement of protoplasm, and the contraction 

 of contractile vacuoles; and finally the movements of cellular 

 organs. 



The simplest case of locomotion is afforded by the amoeboid 

 movement exhibited, among plants, by the zoospores of some 

 Algae and of the Myxomycetes, and by the plasmodia of the 

 Myxomycetes, which are naked masses of protoplasm. Here 

 there are no specialised motile organs, but any part of the 

 protoplasm may be protruded as a pseudopodium into which 

 the remainder of the protoplasm gradually flows, and thus 

 locomotion is effected. In other cases a portion of the proto- 

 plasm is differentiated as a motile organ in the form of one or 

 more delicate filaments known as cilia (see p. i), by the 

 lashing movement of which the organism revolves on its own 

 axis and at the same time travels forward. Ciliary movement 

 is exhibited by the majority of zoospores, by antherozoids 



