THE FRESH-WATER ALGAE 



145 



8i (82) Colony a plate of 4 or 16 spherical cells in a single layer, each with 

 2 cilia. Boundary of gelatinous envelop not distinct. 



Gonium Miiller. 



Cells oval, with two cilia and a pigment spot. Chroma- 

 tophore single, parietal, hollow, with one pyrenoid. Re- 

 production by successive divisions of each cell, forming a 

 new colony; also, according to West, by isogametes. 



Gonium is one of the commonest of the Volvocaceae, 

 occurring in almost all ponds and lakes. It is also one 

 of the most beautiful of the group, as the colonies are ex- 

 ceedingly regular and as they move they revolve, showing 

 first the surface and then the edge. 



Fig. 147. Gonium pectorale Miiller. X 370. (After West.) 



82 (81) Colony flattened, anterior portion rounded, posterior portion with 



three wart-like projections Platydorina Keloid. 



"The two faces compressed so that the cells of the two 

 sides intercalate; flageUa upon both faces on alternate cells. 

 Anterior and posterior poles of major axis are differentiated 

 by the arrangement of the cells and by the structure of the 

 envelope; long and short transverse axes differentiated by 

 the flattening of the colony. Cells .similar, bi-flagellate, 

 each with stigma, chromatophore, and pyrenoid. Asexual 

 reproduction by repeated division of ^1 the cells, each 

 forming a daughter colony." 



Fig. 148. Platydorina caudata Kofoid. X 628. (After Kofoid.) 



83 (80, 88) Colony spherical or spheroidal, but small. Cells not numerous. 84 



84 (85, 86, 87) Colony of 4 or 8 elongated cells with irregular, pseudopodia-hke 



processes, arranged in a zone around the center of a firm 

 gelatinous sphere . Stephanosphaera Cohn. 



Cells elongated, each with ciUa at the anterior pole which 

 penetrate the gelatinous substance. Chromatophores irregular, 

 with one or several pyrenoids. Each cell gives rise to a new 

 colony by division; isogametes are also found. 



Fig. 149. Stephanosphaera pluvialis Cohn. X 425. (After Hieronymus.) 



85 (84, 86, 87) Colony spheroidal, or sUghtly elongated, of 8 or 16 cells closel). 

 packed at the center of the indistinct gelatinous envelop. 



Pandorina Bory. 



Cells heart-shaped, with two ciha at larger end, a pigment spot, 

 and a pyrenoid, the latter in the posterior end of the hollow 

 parietal chloroplast. Reproduction by successive division in 

 each cell whereby as many new colonies are formed as there are 

 cells; reproduction also by the copulation of gametes either alike 

 or slightly unlike as to size; zygospore red. 



Fig. 150. Pandorina morum Miiller. X about 38s. (Original.^ 



