130 Aquatic Organisms 



Ccelastrum is another midsummer plancton alga that 

 forms spherical colonies of from 8 to 32 cells, it has much 

 firmer and thicker cell walls, and the cells are often 

 angulate or polyhedral. New colonies are formed within 

 the walls of each of the cells of the parent colony, and 

 when well grown these escape by rupture or dissolution 

 of the old cell wall. Our figure shows merely the out- 

 line of the cell walls of a i6-celled colony, in a species 

 having angulate cells, between which are open inter- 

 spaces. Kofoid found Ccelastrum occurring in a maxi- 

 mum of 10,800,000 per cubic meter of water in the 

 Illinois River in August. 



Crucigenia is an allied form having ovoid or globose 

 cells arranged in a flat plate held together by a thin 

 mucilaginous envelope. The cells are grouped in fours, 

 but 8, 16, 32, 64 or even more may, when undisturbed, 

 remain together in a single fiat colony. During the 

 warmer part of the season, they are common constit- 

 uents of the fresh-water plancton, the maximum heat 

 of midsummer apparently being most favorable to their 

 development. 



Scenedesmus is a very hardy, minute, green alga of 

 wide distribution. There is hardly any alga that 

 appears more commonly in jars of water left standing 

 about the laboratory. When the sides of the jar begin 

 to show a film of light yellowish-green, Scenedesmus 

 may be looked for. The cells are more or less spindle- 

 shaped, sharply pointed, or even bristle-tipped at the 

 ends. They are arranged side by side in loose fiat rafts 

 of 2, 4 or 8 (oftenest, when not broken asunder, of 4) 

 cells. They are common in plancton generally, espec- 

 ially in the plancton of stagnant water and in that of 

 polluted streams, and although present at all seasons, 

 they are far more abundant in mid and late summer. 



