PRO rOCOCCOIDE.-E 



41 



Order 2.— PROxococcACE.^i (including Palmellace.^^:). 



In this family are included a number of organisms of very simple 

 structure, many of which occur both in the free-swimming {protococcus) 

 and in the resting {pabneUa) condition. In the former state they bear 

 a very .close resemblance to the zoospores of the higher alga^. Other 

 forms are known in one condition only, in which they have a free- 

 swimming motion without the aid of cilia. 



Frotococcus Ag. is one of the commonest objects in fresh water, 

 especially stagnant rain-water, forming masses of a bright green colour, 

 either floating free or attached to a submerged or floating object, but 

 destitute in this state of any spontaneous power of motion. In this, 

 palmella-condition each individual consists of a nearly spherical cell, 

 varying between forty and fifty microns (= "04- -05 mm.j in diameter, 

 which multiphes rapidly by repeated bipartition of its contents. The 

 bright green endochrome has usually 

 intermixed with it a larger or smaller 

 quantity of a red pigment, the propor- 

 tion varying according to the conditions 



-545. — Protococcus pluz'ialis Ktz. 

 A, motile condition ; B, palmella condi- 

 tion ( X 250). (After Cohn.) 



of life, &c. The change to the active 

 condition takes place in the following 

 way. The protoplasm withdraws itself 

 from the cell- wall, and escapes in the 

 form of an ovoid mass provided with 

 two very long and slender vibratile cilia 

 and a pulsating vacuole, by the agency of which it is driven rapidly through 

 the water. The pulsation of this vacuole has been explained by the 

 alternate absorption from the water, through the agency of the chloro- 

 phyll, of carbon dioxide, and the expulsion of free oxygen resulting from 

 the process of assimilation. In some cases the contents of the mother- 

 cell do not escape as a single zoospore, but break up before escaping 

 into eight or more smaller zoospores. The motile protococcus may 

 be either entirely without cell-wall of cellulose, or may have a very 

 dehcate one, through orifices in which the protoplasmic cilia pro- 

 trude. Some observers state that there are two kinds of zoospore in 

 Protococcus — microzoospores and megazoospores, and that conjuga- 

 tion takes place between the latter ; but this last statement at all 

 events requires confirmation. After swimming about rapidly for a 

 time in all directions with an apparently spontaneous movement, the 

 motile protococcus comes to rest, loses its ciha, becomes encysted, or in- 

 vested with a thick cell-wall of cellulose, and again enters the palmella- 



