OF THE CHARACTERISTICS OF THE DIATOMACE^. 



43 



are not only very beautiful objects in themselves, but also 

 of o-reat importance in the determination of species. They 

 are & dependent on the presence of certain ridges, called costae, 

 which are connected with an inner membrane, and also^ of 

 numerous points, more or less fine, which dot the exterior 

 membrane. These points, though sometimes scattered 

 about without order, are more usually seen to be in regular 

 series. Under a low power, the rows of points appear as 

 lines extending across or along the surface of the valve. 

 They are then called striae,* and, in determining species, it 

 is usual to speak of them under that name, although with 

 a very high power most, if not all, of the striae are resol- 

 vable into dots or points. The differences arising from the 

 fineness, approximation, and position of these striae con- 

 stitute the characteristics employed in fixing species. The 

 costa is always visible, even when the valve is immersed in 

 water; but to bring out the striae and points clearly is a 

 much more difficult matter. Before all things the contents 

 of the frustule must be entirely removed, or the latter will 

 not become transparent. Next to that it must be thoroughly 

 dried ; for, if the hollows on the surface of the valve con- 

 tain any moisture, the passage of the light is interfered with, 

 and the markings rendered indistinct, if not invisible. 

 Lastly, the resolution of the striae requires not only a 

 really good microscope, but also extremely careful and de- 

 licate manipulation, especially as regards the illumination 

 of the object. 



In order to dry the frustules quickly and with safety, and 

 to free them from endochrome and other soft substances, they 

 may be exposed for a few seconds to a red heat, which will 

 entirely destroy all organic matter, will dry up any re- 

 maining moisture, and leave the silicious valves bright and 

 transparent. The heating is accomplished by placing a 



* In fig. 48, the costa or vitta, or median line, is the central line 

 marked a,' d. ' The striae are the lines radiating from the centre to 

 the hoop on each side of the costa. The central spot is named the 

 nodule. 



