206 Tetrasporinese 



by a split in the wall of the mother-cell in Ccelastrum (fig. 133 D) and Soras- 

 trum, but by the conversion of this wall into mucilage in Burkillia. 



The Ccelastrese mostly occur in bogs and at the margins of pools and lakes, although 

 some species of Ccelastrum are more particularly confined to the freshwater plankton. 



The genera arc: Ccelastrum Na'geli, 1849 [inclus. Hariotina Dangeard, 1889]; Sorastrum 

 Kiitzing, 1845 [inclus. Sdenosph&rium Cohn, 1879] ; Burkillia W. & G. S. West, 1907. 



Fig. 134. A, Burkillia cornuta W. & G. S. West, x 455 ; a, young autocolonies. B, large 

 ccenobium of Sorastrum spinulosum Nag. x 455. C, Sorastrum Hathoris (Cohn) Schmidle, 

 x 530 (after Bohlin). 



Family Chsetopeltidacese. 



The Algae included in this family are distinguished from all other members 

 of the Tetrasporineae by the setae or bristles which are attached to the cells. 

 The plants are unicellular, or aggregates of cells, sometimes forming flat 

 pseudo-parenchymatous expansions. For the most part they are attached, 

 the cells showing a marked dorsiventrality. 



In Chsetopeltis (fig. 135), an Alga which lives attached to various aquatic 

 macrophytes, the ' thallus ' is flat and similar to that found in the Ulvacea? 

 except for its small size, and the fact that it is attached by the whole of one 

 surface. A study of the development of this ' thallus ' shows that it is not a 

 concrescence of branches as usually described, but that cell-division may 

 occur with considerable irregularity in any direction in one plane. The cells 

 are embedded in a firm jelly, and simple elongated bristles usually grow out 

 from the cell-walls of some of them. Reproduction generally takes place by 

 quadriciliated zoogonidia, but biciliated isogametes are also known to occur. 



Ch&tosph&ridium (fig. 136 A C) is much the most frequent genus and is 

 found in stagnant water attached to various larger Algae, or sometimes free- 

 floating and unattached. The cells generally form irregular aggregates 1 , 

 sometimes enveloped in a copious mucus (Ch. Nordstedtii), at other times 



1 The statement that the cells of Chsetosphseridhtm when mature are ' united by longer or 

 shorter empty tubes to form a sympodial branch-system' requires some qualification. The 

 author has examined hundreds of mature British and European specimens of this genus, and in 

 no instance had such tubes been formed. The division of the cells was transverse, the lower 

 cell simply sliding out during growth from under the upper one. 



