372 ELEMENTARY LIOLOGV. [CIIAP. 



It is of interest to note that in tlie case of small vege- 

 table organisms the cellulose coats have been at times 

 observed to be thrown out in a distorted, if not a digested 

 and disintegrating condition. 



The chemical composition of the bodies of the Avioclue 

 has not been accurately ascertained, but they undoubtedly 

 consist, in great measure, of water containing a protein com- 

 pound, and are similar to other forms of protoplasm. They 

 absorb oxygen and give out carbonic acid, and the presence 

 of free oxygen is necessary to their existence. When the 

 medium in which they live is cooled down to the freezing 

 point their movements are arrested, but tliey recover when 

 the temperature is raised. At a temperature of about 35" C. 

 their movements are arrested, and they pass into a condition 

 of "heat-stiffening," from which they recover if that tem- 

 perature is not continued too long; at 40" to 45° C. they 

 are killed. 



Crystals are generally to be met with in the cell proto- 

 plasm, but of their origin and significance nothing is at 

 present known. 



Electric shocks of moderate strength cause Afucebce at 

 once to assume a spherical still form, but they recover after 

 a while. Strong shocks kill them and, at the same time, 

 bring the nucleus distinctly into view. 



If Ama'biE are not to be found, their nature may be 

 understood by the examination of the colourless corpuscles of 

 the blood. (Cf. ch. I. pp. 121 — 123.) 



The colourless corpuscles of the blood of some of the 

 cold-blooded vertebrates, such as Frogs and Newts, may be 

 kept alive for many weeks in serum properly protected from 

 evaporation; and if fmely divided colouring matter, such 

 as indigo, is supplied to them, either in the body or out of 

 it, they take it into their interior in the same way as true 



