USE OF THE MICROSCOPE IN ZOOLOGY. 37 



which you have collected some specimens. I have 

 generally found Volvox globator in stagnant ponds 

 containing a profusion of aquatic vegetation. The 

 ordinary size is about -^ of an inch in diameter. 

 " When examined with a sufficient magnifying power, 

 the volvox is seen to consist of a hollow sphere, 

 composed of a very pellucid material, which is 

 studded at regular intervals with minute green spots, 

 and which is often, but not constantly, traversed by 

 green threads connecting these spots together. From 

 each of the spots proceed two long cilia; so that 

 the entire surface is beset with these vibratile fila- 

 ments, to whose combined action its movements are 

 due. Within the external sphere may generally be 

 seen from two to twenty other globules of a darker 

 colour and of varying sizes ; the smaller of these are 

 attached to the inner surface of the investing sphere, 

 and project into its cavity; but the larger lie freely 

 within the cavity, and may often be observed to revolve 

 by the agency of their own ciliary filaments. After 

 a time the original sphere bursts, and the contained 

 spherules swim forth, and speedily develop themselves 

 into the likeness of that within which they have been 

 evolved." When you see, as you will do, various 

 organisms swimming freely about in a drop of water, 

 you will be inclined to put them down as belonging to 

 the animal kingdom. Suspend your judgment; it is 

 quite probable what you see are motile cells of certain 

 vegetable organisms. What is the difference between 

 a plant and an animal ? 



* Carpenter, " Microscope," p. 251. 



