GROUP I. THALLOPHYTA: ALGJi : CHLOKOPHYCE^. 



237 



and, surrounding itself with a cell-wall, directly constitutes itself a new 

 individual. 



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 multi- 

 cellular 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) aplauogamete with a larger (female) aplanogamete ; the repro- 

 ductive cells (planogametes or aplanogametes) are formed by division ; the 

 zygospore represents the sporophyte, and generally gives rise, on germination, 

 to two or four motile or non- 

 motile individuals. 



The principal genera are Chla- 

 raydomonas, Haematococcus (or 

 Sphaerella), Pbacotus. 



Order 2. Volvocaceae : body 

 multicellular, consisting of a de- 

 finite or an indefinite number of 

 cells, which may be all alike 

 (ccenobium), or there may be a 

 distinction between vegetative and 

 reproductive cells ; vegetative re- 

 production, by division of any or 

 all of the cells of the body, or of 

 certain special gonidial cells (Vol- 

 vox), from each of which a new 

 individual is formed ; sexual re- 

 pi eduction, isogamous by piano- 

 gametes, or oogamous. 



The order may be divided into 

 two families in accordance with the nature of the sexual process : 



Fam. 1. Pandorinea: : sexual process isogamous; body a ccenobium consisting 

 of a definite number (usually 16 in Gonium and Pandorina, and 8 in 

 Stephanosphffira) of cells; any of the cells may divide to form a new individual, 

 or to form planogametes ; the zygospore (at least in Pandorina) representing 

 the sporophyte, sets free one or two zoospores on germination. 



The body of Pandorina is a solid sphere (Fig. 167) ; that of Gonium is a 

 flat, square plate of cells ; that of Stephanospluera consists of a zone of cells 

 lying on the equator of the more or less nearly spherical mucilaginous common 

 cell-wall. 



Fam. 2. Eudorinecc : sexual process oogamous ; body, a hollow sphere, either 

 a ccenobium consisting of a definite number (geuernlly 32) of cells (Eudorina) ; 

 or consisting of a very large number of cells, some of which are specially 



FIG. 107. Pandorina Mo rum (x400). A vege- 

 tative stage. B two planogametes in process of 

 conjugation. 



