1892.] MICROSCOPICAL JOURNAL. 33 



which is with- difficulty discerned at all. In the diatoms the enclosing 

 case will prove to be a solid deposit of nearly pure silica ; but in the 

 desmids it will be found to be a semi-flexible film sliglitly infiltrated 

 with carbonate of lime. This is an important distinction, which forms 

 the basis for a sharp separation of the two groups of forms. As regards 

 the particular specimens we are supposed to have under immediate in- 

 spection, however, we shall by this time have observed a difierence 

 which to most minds will constitute a clearer demarcation than could 

 be established bv any other criterion ; and that is that our desmids are 

 apparently quite passive, while the little boat-shaped diatoms actively 

 glide about with a seemingly well-defined purpose. 



Locomotion. — You will need to abstain from generalizing too far 

 and too fast in this connection, for all diatoms do not possess this power 

 of locomotion, since not only are some of them united to one another, 

 as we have seen, but many sorts are attached to plants and other fixed 

 objects, although the theory has been advanced that all diatoms are, at 

 one time or another, free and migratory. On the other hand, while 

 desmids are never permanently anchored, locomotion is not a promi- 

 nent characteristic of even the solitary forms, because, while it is pretty 

 general througliout the group, it is too slow to attract attention, and, 

 when ol)served. has not that appearance of voluntariness which is ex- 

 ceedingly striking in the movements of the free, boat-shaped diatoms. 

 When you have a specimen of the latter under your microscopically- 

 aided eye, you will see it slide over the bottom of your artificial pond 

 without discernible machinery but with very appreciable force and con- 

 siderable speed. If it meets an obstruction which is not too large, it 

 pushes it aside, but, if too heavy, it halts a few seconds and then re- 

 verses its invisible engine and changes its course. 



The diatom possesses a body of plastic substance called 

 protoplasm. — This, upon close examination, we easily learn to be the 

 case. Very simple methods of investigation show that every frustule 

 (as each individual cell is called) is in reality a glass box enclosing a 

 semi-fluid, nearly transparent, but somewhat granular material, con- 

 taining, besides the oily-looking spots or vacuoles to whicli I have be- 

 fore referred, more or less of a yellowish or brownish-green substance, 

 which gives its general color to the whole organism. There is reason 

 to believe that the semi-fluid material also forms an enveloping layer on 

 the outside of the siliceous carapace of all diatoms, and in many in- 

 stances a similar glycerine-like matter permanently encloses a whole 

 colony of otherwise separate frustules, which passively spend their ex- 

 istence within its restraining grasp. Now, chemical tests disclose the 

 fact that the colloidal mass within the transparent valves is composed 

 of carbon, oxygen, hydrogen, and nitrogen in proportions and relations 

 which characterize an organic substance, and a great number of con- 

 verging facts, derived from long-continued observation, go to prove that 

 this substance is the seat of all the changes, chemical and physical. 



It is through and by this protoplasm that the diatom responds to the 

 influences of heat and light, that it receives and assimilates its food, that 

 it moves from place to place as we have seen the boat-like specimen in 

 the Croton water doing, that it grows and reproduces its kind. It was 

 because of its agency in these matters that Prof. Huxley called proto- 

 plasm " the physical basis of life," though its name really means 



