Movements 



99 



assumed by diatom colonies are to be accounted for very largely by Un- 

 varying position of the secretory pores in the different genera and species. 



The colonies of Asterionella, Meridian, and Eucampia are particularly 

 noteworthy for the beautiful disposition of the cells composing the colony. 

 In the first-named genus the elongated frustules are united by their corners 

 in such a manner that they radiate from a central circle like the spokes 

 of a wheel (fig. 73.4); in the other genera an unequal growth of the girdle 

 of each cell results in the development of spiral ribbons. In the freshwater 

 plankton one of the abundant forms of Tabellaria fenestrata assumes this 

 habit and has been named T. fenestrata var. asterionelloides (fig. 73 B). 



Fig. 72. Parts of the valves of various diatoms showing the pores through which mucus is 

 secreted. J, Diatotna yramle W. Sm. B and C, Grammatophora serpentina Kiitz., girdle 

 and valve views respectively. D, Synedra Ulna (Nitzsch) Ehrenb. var. splendens (Kiitz.) 

 V. Heurck. E, Fratjilaria virescens Ralfs. F, TabeUaria fenestrata (Lyugb.) Kiitz. p, pore. 

 All x 2200. (After 0. Miiller.) 



In this diatom, and in the plankton-species of Asterionella, Voigt ('01) states 

 that a gelatinous membrane can be detected stretched between the radiating 

 cells of the colony, thus considerably augmenting their floating capacity. 



THE MOVEMENTS OF DIATOMS. Many of the free, unattached diatoms 

 exhibit movements which even yet are but little understood. In some 

 species the movements are relatively slow, but in others they are much 

 quicker, and the individuals can be seen propelling themselves backwards 

 and forwards in the direction of their longer axis. This movement is generally 

 of a jerky character, although more rarely creeping and steady. It has been 

 suggested that it only takes place when one valve-face of the diatom is in 

 contact Avith some kind of substratum, but it must be admitted that there 



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