. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . alve (fig. *J2F), in Diatomathere is always one pore towards the end of the valve in a somewhat asym-metrical position (tig. 72 ^4). In Synedra and Fragilaria the pores are alsonear the extremities of the valves (fig. 72 D and E). The different forms Movements 99 assumed by diatom colonies are to be accounted for very largely by thevarying position of the secretory pores in the different genera and species. The colonies of Asterionella, Merid
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . alve (fig. *J2F), in Diatomathere is always one pore towards the end of the valve in a somewhat asym-metrical position (tig. 72 ^4). In Synedra and Fragilaria the pores are alsonear the extremities of the valves (fig. 72 D and E). The different forms Movements 99 assumed by diatom colonies are to be accounted for very largely by thevarying position of the secretory pores in the different genera and species. The colonies of Asterionella, Meridian, and Eucampia are particularlynoteworthy for the beautiful disposition of the cells composing the the first-named genus the elongated frustules are united by their cornersin such a manner that they radiate from a central circle like the spokesof a wheel (fig. ); in the other genera an unequal growth of the girdleof each cell results in the development of spiral ribbons. In the freshwaterplankton one of the abundant forms of Tabellaria fenestrata assumes thishabit and has been named T. fenestrata var. asterionelloides (fig. 73 B)..
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