STRUCTURE OF VOLVOX GLOBATOR. 



251 



the cycle of changes occurs which has been already described ; and 

 the Plant may pass through a long series of these, before it returns 

 to the state of the red thick-walled cell, in which it may again 

 remain dormant for an unlimited period.— Even this cycle, how- 

 ever, cannot be regarded as completing the History of the species 

 before us ; since it does not include the performance of any true 

 Generative act. There can be little doubt that, in some stage of 

 its existence, a Conjugation of two cells occurs, as in the pre- 

 ceding case ; and the attention of observers should be directed to its 

 discovery, as well as to the detection of other varieties in the condi- 

 tion of this interesting little Plant, which will be probably found 

 to present themselves before and after the performance of that act. 

 193. From the Composite ' motile ' forms of the preceding 

 type, the transition is easy to the group of Volvocinece, — an assem- 

 blage of minute Plants of the greatest interest to the Microscopist, 

 on account both of the Animalcule-like activity of their movements, 

 and of the great beauty and regularity of their forms. The most 

 remarkable example of this group is the well-known Volvox glo- 

 bator (Fig. 108), which is not uncommon in fresh-water pools, and 

 which, attaining a diameter of l-30th of an inch, may be seen 

 with the naked eye when the drop containing it is held-up to the 

 light, swimming through the water which it inhabits. Its onward 

 motion is usually of a rolling kind; but it sometimes glides 

 smoothly along, without turning on its axis ; whilst sometimes, 

 again, it rotates like a top, with- 

 out changing its position. When 

 examined with a sufficient magni- 

 fying power, the Volvox is seen to 

 consist of a hollow sphere, com- 

 posed of a very pellucid material, 

 which is studded at regular inter- 

 vals with minute green spots, and 

 which is often (but not constantly) 

 traversed by green threads con- 

 necting these spots together. 

 From each of the spots proceed 

 two long cilia; so that the entire 

 surface is beset with these vibra- 

 tile filaments, to whose combined 

 action its movements are due. 

 "Within the external sphere may generally be seen from two to 

 twenty other globes, 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 de- 

 velope themselves into the likeness of that within which they have 



Fig. 108. 



^fttssm 



Volvox Globator, 



