44O PHYSICAL GEOGRAPHY. 



in a few species, and those seemingly the larger, the motion is a steady 

 progressive one, yet it is by far commoner to find that it is unsteady and 

 trembling, as if it were the tottering steps of the infant, or of extreme 

 old age. Navicula is one of those genera which are usually classed 

 among the free forms, and in them all, with perhaps one or two excep- 

 tions, when they have any progressive motion, it is that which we have 

 described. There is a genus which has very much the form of a horse 

 saddle with the two flexures, and known as Campylodiscus, in which "the 

 motion never proceeds farther than a languid roll from one side to the 

 other." 



As has been remarked, the earlier observers of these atomies, being 

 insufficiently informed on the subject of the economy of the vegetable 

 kingdom, considered the possession of the power of spontaneous motion 

 by any being indisputable evidence of its animal nature ; and, on this 

 foundation, it became easy to rear up a mass of proofs that the diatoma- 

 ceae were certainly animals. The space within the two-valved shell, like 

 an oyster or clam, was the animal matter furnished with special organs, 

 if not with muscles, by means of which the movements were accom- 

 plished. Of course, the many clear vacuole-like-looking spots of oily 

 matter were the stomachs, and, with the imperfect microscopes of the 

 time, observers were (they thought) able to see the protrusion of a "foot" 

 like that upon which the snail travels, through the central portion, which 

 looked to them like a round opening, but which we now know to be a 

 thickened portion of the shell. A late observer has asserted that he has 

 seen, along the so-called "median line," an appearance indicating the pro- 

 trusion of an organ or series of organs of some kind ; but as others, 

 equally competent microscopists, have not been able to satisfy themselves 

 that there are such organs, this gentleman's assertion can hardly be said 

 to be proven. There are still a few microscopists who hold to the belief 

 in the animal nature of the diatomaceae, although by far the majority 

 rank them as plants. One gentleman in England says that he has 

 seen (and, what is more, figures) the cilia, or hairs, which move about 

 like arms, and by means of which these creatures change their place. 

 Unfortunately, he first takes the animal nature of the diatomaceae for 

 granted, and then attempts to prove the existence of the cilia as organs 

 of locomotion. 



