GROUP I. THALLOPHYTA : ALG.E. 



249 



forms (e.g. Tetraspora, Chlorochytrium). The zygospore, on germination, 

 usually gives rise to one or two zoospores. 



Series II. VOLVOCOIDE.E. The body, in this series, is free-swimming 

 for at least a considerable portion of its life, and consists of one or more 

 cells clothed with a somewhat mucilaginous cell- wall, through which 

 the cilia (usually two from each cell) project into the water. According 

 to the unicellular or multicellular structure of the body, two orders may 

 be distinguished : 



Order 1. Chlamydomonadaceae : body a single cell, resembling in 

 many cases a zoospore in appearance, but differing essentially from it in 

 possessing a cell-wall ; some forms have a resting Palmella-stage, in 

 which they multiply by division ; sexual process, generally isogamous 

 with fusion of planogametes, but sometimes in Chlamydomonas it is really 

 oogamous, consisting in the fusion of a small (male) aplanogamete with a 

 larger (female) aplanogamete ; the reproductive cells (planogametes or 

 aplanogametes) are formed by division ; the zygospore generally gives 

 rise, on germination, to two or four motile or non-motile individuals. 



The principal genera are Chlamydomonas, Hsematococcus (or Sphae- 

 rella), Phacotus. 



Order 2. Volvocaceae : body 

 multicellular, consisting of a 

 definite or indefinite number of 

 cells, which may be all alike 

 (coenobium), as in Pandorina 

 (Fig. 138), which consists of 

 16 cells ; or there may be a 

 distinction between vegetative 

 and reproductive cells (Volvox). 

 Vegetative reproduction is ef- 

 fected by division of any or all 

 of the cells of the body (Pandor- 

 ina), or of certain non-motile 

 gonidial cells (Volvox), from 

 each of which a new individual 

 is formed; sexual reproduction, 

 isogamous by planogametes 

 (Pandorina), or oogamous (Vol- 

 vox). 



In Pandorina any cell may divide to form a new individual, or to forni 

 planogametes ; the zygospore sets free one or two zoospores on germi- 

 nation. 



Volvox is sometimes monoecious, sometimes dioecious. The contents of 

 the oospore undergo repeated bipartition until the requisite number of 

 cells to form a new individual is attained. The vegetative development 

 of new individuals from the gonidial cells of Volvox, takes place in pre- 

 cisely the same way as the development from the oospore : the small 

 individuals formed vegetatively from the gonidial cells are set free into 

 the cavity o.f the parent, where they remain until it dies, when they are 



Pis. 138. Pandorma Morum ( x400) : A vege- 

 tative stage ; B two planogametes in process of 

 conjugation. 



