216 



Chlorococcineze 



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 zoogonidiuin. 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 viridis Schmitz. 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.) 



Halosphsera 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 Spht&ra Kerguelensis Karsten, described 

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

 N. Atlantic, are developmental stages of Halosphsera. 



Family Hydrodictyaeese. 



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

 ccenobia of coenocytes, floating freely in the water. The coenocytes, 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 coenobium 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 coenobium of Hydrodictyon they are all 

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

 a large irregular colony. 



