STRUCTURE AND LIFE-HISTORY OF VOLVOX GLOBATOR. 41 



stimulate the action, while cold and darkness retard it. The ciliary 

 action is affected in a remarkable deforce by altered external conditions. 

 If a drop of water considerably colder than that in which the Volvoces are 

 floating be allowed to flow in under the cover-glass, the whole are 

 paralysed for some seconds, after which they slowly resume their motion. 

 A sudden mechanical shock produces a similar effect, as I have repeatedly 

 seen to happen in consequence of the rude impact of some ferocious 

 Entomostracon. 



A sufficient degree of heat to make the water distinctly tepid to the 

 feel, causes instant and simultaneous death of the whole colony. 



During the day the majority of the Vulvoces contained in a shallow 

 vessel rise to the surface, although they avoid strong direct sunshine, 

 while at night they retire in a cloud to the bottom. 



The astonishing number in which the spheres at times appear in 

 some pool and their equally sudden disappearance, have been frequently 

 remarked. Doubtless a very slight change in external conditions suffices 

 on the one hand to favour the development of countless thousands of 

 young plants, and on the other, either to destroy the vitality of the whole 

 colony, or to drive it to seek refuge in deeper water. 



A curious instance of this sensitiveness to varying conditions of light 

 and heat occurred to myself. I had two shallow vessels in a north 

 window, each containing a goodly supply of Volvox. Cold and inclement 

 weather, which prevailed for weeks together, seemed to check their 

 increase, for I found but few young spheres from day to day among the 

 older ones. Thinking that a moderate degree of warmth would tend to 

 increase my colony, I transferred one vessel, fortunately not both, to the 

 floor of a warm greenhouse. In forty-eight hours all were dead, and in 

 a few days scarcely a vestige remained of the countless corpses which had 

 copiously strewed the bottom of the glass. 



We must now revert to the minute structure of the mature parent- 

 sphere, which has been exliaustively studied by Cohn, Busk, and 

 Williamson. 



In the outset it should be stated that the last-named observer 

 believes that there are two distinct forms of Volvox, in one of which the 

 peculiar structure which I am about to describe exists, while it is 

 absent from the other. Busk disputed the the accuracy of Williamson's 

 observations on this point, but, in an appendix published subsequent to 

 the body of his essay, he states that he has detected this same structure 

 in specimens from Manchester, but iiot in his own. 



I have failed to develop it by the means recommended by Williamson, 

 but have succeeded in making it evident enough in a great number of 

 specimens from Sutton, by the use of other re-agents, and especially 

 by the application of aniline purple, an invaluable auxiliary in the 

 examination of minute vegetable cell-structures. 



This substance stains the protoplasmic elements of such structures 

 to a colour which appears deep purple by direct light, and crimson by 



