20O 



DIATOMACEJE, 



PART II. 



Fig. 14. Actinocyclus undulatus. 



shells. 4 From the few examples given, a faint idea 

 only can be formed of the variety and beauty of the 

 engravings on the diatoms. It had long been doubted 

 whether those on the valves of Coscinodiscus, Triee- 

 ratium and others, were elevations or depressions, but 

 Professor Eood of New York, tlnited States, has proved 

 them to be depressions by an optical arrangement which 

 will be useful for the investigation of microscopic forms. 

 Diatoms increase by spontaneous bisection, by conju- 

 gation, and by the resolution of their endochrome into 



minute spores, 9alled go- 

 nidia. When bisection is 

 about to take place the cell 

 elongates, the hoop increases 

 in breadth, the endochrome 

 divides into two equal parts, 

 and the coating of the 

 cell bends in between them, 

 which gives the diatom the appearance of an hour- 

 glass. At last they separate, and upon each of the 

 new surfaces a new silicious half is formed, usually 

 the exact counterpart of the old one, so that there are 

 two diatoms instead of one ; and the process may be 

 continued indefinitely. In most cases, the new diatoms 

 thus produced are free and independent. Sometimes, 

 however, they adhere to one another by a fragment or 

 connecting membrane, and if they happen to be slender 

 and rectangular, and attached side by side, they form a 

 slender filament, or if attached by alternate angles they 

 form a zigzag chain, as in fig. 11. 



The Meridion circulare (fig. 15) is a diatom of exquisite 

 beauty, millions and millions of which cover every sub- 

 merged stone, twig, or blade of grass, and even form the 



* The author is indebted throughout many parts of this section to the 

 excellent work of Dr. Carpenter, and to the ' Cryptogamic Botany ' of the 

 Eev. M. J. Berkeley, from which also many of the cuts are derived. 



