216 



Chlorococcineae 



nuclei remaining in a peripheral position. The cytoplasm and chloroplasts then 

 become aggregated around each nucleus, forming bodies of a plano-convex 

 character which after division of the nucleus become constricted into two parts, 

 each part forming a zoogonidium. Sometimes the development of motile repro- 

 ductive cells is arrested, so that the divisions of the original protoplast result in 

 the formation of aplanospores (generally 16 in the mother-cell ; Cleve, '98). 



Fig. 142. Halosphsera riridis Scbmitz. 1 and 2, vegetative cells. 3, part of the peripheral 

 plasma during the formation of zoogonidia ; chr, chloroplast ; pi, cytoplasm ; k, nucleus. 

 4, zoogonidium. (After Gran and Schmitz, from Oltmanns.) 



Halospheera viridis Schmitz is really an inhabitant of the warmer tem- 

 perate seas, although in the Atlantic Ocean it is carried northwards by the 

 Gulf Stream. It is not unlikely that Splnrra Kerguelensis Karsten, described 

 from the Antarctic Ocean, and Pachysphwra pelagica Ostenfeld, from the 

 N. Atlantic, are developmental stages of Halosphsera. 



Family Hydrodictyaceae. 



In this family of the ChlorococcineaB the plant-body consists of non-motile 

 ccenobia of ccenocytes, floating freely in the water. The ccenocytes, which 

 are of very varied external form, are disposed so as to form a flat plate in 

 Pediastrum and arranged in the manner of a net in Hydrodictyon. In the 

 disc-like ccenobium of Pediastrum, in which there may be more than 100 

 ccenocytes, those of the marginal series differ in external characters from those 

 in the centre ; but in the net-like ccenobium of Hydrodictyon they are all 

 of the same cylindrical shape and many hundreds may be united to form 

 a large irregular colony. 



