PROFESSOR W. C. WILLIAMSON ON VOLVOX GLOBATOR. 833 



each of the individuals becomes ruptured, when the fluid 

 and granular cell-contents flow out ; they mingle freely 

 with the water and with each other, but never regain their 

 primary spherical contour. 



From the foregoing outline of the principal facts pre- 

 sented by the Volvoiv ghhator, we may now proceed to 

 consider its probable position in the kingdom of nature. 



I am aware, that prior to arriving at a conclusion as to 

 whether it is an animal or a vegetable, it will be expected 

 that I should define what I comprehend in each of these 

 terms. I confess myself unable to do this satisfactorily. 

 The attempt has frequently been made by others ; but 

 none of their definitions are free from objections, or embrace 

 all the numerous deviations from the typical forms of each 

 kingdom. The most plausible distinction between plants 

 and animals, is that which assigns to the former the power 

 of assimilating inorganic mineral food; whilst the latter can 

 only take into their systems that which is already organized. 

 It is probable that this distinction is a valid one ; but it is 

 scarcely one of practical application as a test of special 

 moot cases. If the digestive process involved the necessity 

 for an internal digestive cavity with an external oral orifice, 

 the case would be different ; but there is every reason to 

 believe, that some examples of animal organisms receive no 

 food into internal cavities, but are endowed with a power of 

 embracing the object on which they are about to feed, and 

 thus absorb nutriment from the body with which they are 

 merely in contact. The AriKsba is still a case in point — 

 even though we should concede the vegetability of marine 

 and fresh-water sponges, which I am not prepared to do. 



M. Agassiz and Dr. Gould, in their recently pubhshed 

 Principles of Zoology, speak unhesitatingly on this point. 

 They enumerate " distinctly limited cavities, destined for 

 the lodgement of certain organs," as existing "in all ani- 

 mals without exception." They also say, that " the well- 



