Ch. XJ 



THE DIATOMS 



429 



*i 



n 





rn 





it again forms the valves characteristic of its kind. Sexual 

 reproduction occurs through conjugation of two protoplasts, 

 and formation of zygospores, with various differences in de- 

 tails in the various kinds, although in a very few cases 

 ciliated zoospores occur, suggestive of the Flagellates. 

 They spread in small part by free locomotion through aid 

 of protoplasmic threads working against the ground, but 

 chiefly are drifted with movements of the water. At times 

 they form in the ocean great clouds, 

 sometimes with distinctive tinges of 

 color ; and in such state they serve 

 as food for various fishes. A large 

 number of species is known, probably 

 6000, with others which are fossil ; 

 but they have no direct economic 

 utility aside from the uses above 

 mentioned, and a certain limited 

 employment as test objects for mi- 

 croscopic lenses, to which purpose 

 they are fitted by the extreme fine- 

 ness and regularity of their markings. 

 Ecologically the Conjugal es are 

 chiefly drifting micro-hydrophytes. 

 Phylogenetically, the Zygnemacese 



are obviously related to the Pro- of the cell. (Adapted from 

 tococcales, of which they seem fitzer -) 

 merely a specialized branch (Fig. 275). The Desmids in 

 turn are clearly related to the Zygnemaceae, the lower forms 

 of both being very much alike. The position of the Diatoms 

 is less certain, for no connecting links with other groups 

 exist; but their mode of reproduction suggests an origin 

 from the Desmids, as a very ancient offshoot. 



Fig. 297. — A typical Dia- 

 tom, Navicula viridis, much 

 magnified. 



Left, a face view; next, 

 profile in optical section, 

 showing arrangement of the 

 two valves of the wall ; and, 

 right, the mode of division 



Order 4- Siphon ales: the Tubular Aloje. These are 

 mostly marine forms of warm seas, but include some felt- 

 like or vesicular kinds which grow on the mud of shoal ponds 



