NATURAL HISTORY OF THE DIATOMACEAE. 439 



W. Smith, the author of the Synopsis of the British Diatomaceae, among 

 that number, have classified them in such a way as to constitute two 

 great groups, namely, those which are free, and those which occur 

 attached. It would seem most natural that those forms which commonly 

 present themselves attached should become free at some period of their 

 existence for the purpose of disseminating the species, for we do not find 

 that the diatomaceae produce seed which may be wafted about by the 

 motion of the water, or young which are specially characterized by pos- 

 sessing organs of motion, so that this phenomenon may be accomplished. 

 However this may be, it is a fact that many of them possess peculiar 

 movements, produced by and inherent in themselves, and which have 

 from the earliest times, when they were first observed, attracted the 

 attention and aroused the wonder of possessors of microscopes. And 

 when these seeming sentient movements are watched by means of the 

 magnifying glass, it is not to be wondered at that many observers have 

 been disposed to class them among animals possessing complicated organs 

 of locomotion, digestion, and reproduction, if not reasoning powers to 

 guide and direct those organs. And this power of active movement is 

 not confined to those forms alone which are usually seen free, for many 

 of the fixed forms, if detached from their support, will immediately take 

 upon themselves motions precisely similar to those of their normally 

 free brethren. 



The extreme liveliness of some of the diatomaceae has been considered 

 by many as proof of their animal nature ; but when we know that the 

 seeds of many large and acknowledged plants growing in water, namely, 

 the algae, are even more active in their motions than our little friends, and, 

 as in the case of those plants, this motion is evidently for disseminating 

 the species, so we may naturally believe that some similar provision is 

 made for the wide spreading of the diatomaceae. If we watch, by means 

 of a good microscope, an individual belonging to the genus Navicula, in 

 which the form, when viewed in the direction which is usually presented 

 to the eye, is that of a double-prowed boat, something like an Indian 

 canoe, we find that it moves after the manner of a boat, but with either 

 prow forward, as happens to be most convenient, apparently, for, after 

 moving in one direction for a few seconds or minutes, it will immediately 

 return upon its course, now propelling the other forward. And although 



