58 HOW TO USE THE MICROSCOPE 



one valve fitting over the other like a lid on a pill- 

 box. A slimy secretion covers the exterior of the 

 cell wall, and this enables the little plant to slip 

 easily over the surface of the mud. It is exceedingly 

 interesting to see diatoms gliding quite majestically 

 on the glass slip under the microscope. The cell 

 walls are covered with wonderful markings. Live 

 diatoms are coloured with chlorophyll, the usual 

 colouring matter of green plants. But diatoms are 

 also found in fossil forms, from which the colouring 

 matter has, of course, disappeared. There is a vast 

 variety of living and fossil species, and some micros- 

 copists devote their whole attention to their study. 

 Preliminary examinations of diatoms should be 

 made with a fairly low power — § inch or J inch — but 

 to see them in fuller detail a J-inch or ^-inch 

 objective must be used. They vary considerably in 

 size. 



Other single-celled plants, called " Desmids," may 

 be taken from ponds, especially such as have green- 

 looking water, either with the net or scraped from 

 the bottom. These tiny plants exhibit interesting 

 protoplasmic activity within their cell walls. But 

 I must pass rapidly to a brief consideration of a few 

 forms of distinctly animal life common in the pond. 



In sweeping the water with our net, we are fairly 

 sure to capture a quantity of Water -Fleas, which we 

 shall see swimming in the bottle in a jerky fashion. 

 By means of a dipping tube (p. 11) we transfer one 

 of these lively creatures to the live cage, and 

 examine it with a low power. We find that the 

 animal, which is hardly more than yV inch long, is 

 quite highly organized. It has a small head, with 



