2] EFFECT ON METABOLISM AXD MOVEMENT 99 



cola. When Actinosphserium is subjected to a slight stimula- 

 tion, such as would be produced by other Protozoa wandering 

 among its pseudopodia, it shows no response. But when an 

 infusorian or a rotifer swims against the pseudopodia with 

 force, they discharge a sticky substance which holds the dis- 

 turbing organism fast. The same result follows the irritation 

 of one of the pseudopodia by touching it with a fibre of cloth 

 or filter paper. Like effects follow the irritation of Thalas- 

 sicola. Thus, some Protista respond to particular kinds of 

 contact by the excretion of a sticky substance. 



In the higher animals, also, contact may call forth secretions ; 

 thus, the stolons of many hydroids secrete a cement from the 

 surface applied to the substratum. 



Among the higher plants, also, contact has sometimes a similar 

 effect. Examples appear in DARWIN'S ('75, p. 393) work on 

 the gland cells of insectivorous plants. In many species, to 

 be sure, e.g. Drosera, Dionsea, Drosophylluin, mere contact of 

 inorganic bodies has no effect upon the secretions of the glands 

 of the leaves. In the case of Pinguicula lusitanica, however, 

 fragments of glass, as well as seeds and albumen, caused the 

 glands with which they came in contact to secrete more freely 

 than before. 



This response to contact by secretion is, for the most part, 

 an advantageous one. It enables the Protista and the insec- 

 tivorous plants to hold their prey or their enemy, as the case 

 may be ; and it enables the stolon to hold fast to the sub- 

 stratum. 



The change in metabolism may be so profound as to lead 

 to death. HOKVAKTH ('78) and MELTZER ('94) have shown 

 that when bacteria are violently shaken, not only is growth 

 interfered with, as we shall see in the second part of this 

 book, but death may ensue, so that cultures of bacteria may 

 be sterilized. 



We now turn to consider the modification of movement by 

 molar agents. The general phenomena are familiar. An 

 amoeba, any other rhizopod, or a white blood corpuscle con- 

 tracts when the cover-glass over it is disturbed. The stream- 

 ing in the plasmodia of Myxomycetes is retarded or inhibited 



