444 PHYSICAL GEOGRAPHY. 



mean one bundle of individuals) slide out to its utmost limit across the 

 field of view, and then, becoming entangled with others, which likewise 

 were made up of many individuals, some eight or ten of its frustules 

 were twisted around, almost off from the rest, so as to lie at right angles 

 to them ; and when the group containing the largest number of frustules 

 receded to their former position, which they soon did, the eight or ten 

 seeming, by the act of twisting, to lose their power of motion among 

 themselves for the time being, were dragged along in a helpless condi- 

 tion, and twisted completely around one revolution, so as thereafter to 

 fall back again into their places, when all went on again as usual, that 

 is to say, the regular motion of all the frustules over each other succeeded. 

 Now what kind of a joint can it be that permits of such eccentric move- 

 ment ? The motion of all diatoms is accelerated by a moderate heat, so 

 that specimens gathered during the winter months, and remaining either 

 quiescent, or only exhibiting very slight motion when viewed by means 

 of the magnifying glass of the microscope, may be made to move vigor- 

 ously by the cautious application of warmth, as by placing them in a 

 warm room, or by holding the glass slide, upon which they are, upon the 

 palm of the hand for a short time. 



PART THIRD. 



Mode of Growth of the Diatomace^e. 



When we speak of the growth of the Diatomace?e, it must not be 

 confounded with the reproduction of the organisms, although modern 

 physiologists are coming to understand that growth or increase in vol- 

 ume, and reproduction or increase in number, are very nearly related, if 

 they be not but modification, in degree and in direction, of a force acting 

 within and essential to the existence of living beings. 



We have seen what the structure of the outer coat or siliceous skeleton 

 of the diatomaceas is. Let us now, before asking how they grow or 

 increase in dimensions, learn what is known with regard to their internal 

 economy, that is to say, the anatomy of their softer parts. Unfortu- 

 nately, on this point our knowledge is extremely unsatisfactory, and it 

 even appears, as has been already hinted, that the result of modern 

 investigations would be to upset a great deal of what we have up to a 

 late date considered as settled in connection with this point. 



