146 



FRESH-WATER BIOLOGY 



86 (84, 85, 87) Colony spherical or ellipsoidal; cells of two types, vegetative and 

 gonidial, which lie in the anterior and posterior parts of 

 the colony respectively Pleodorina Shaw. 



Colony consists of a spherical or elliptical coenobium of 

 greenish, bi-flagellate cells of two types, vegetative and 

 gonidial, in the anterior and posterior parts of the colony 

 respectively which lie in the periphery of a hyaline gelatinous 

 matrix and are surrounded by a common hyaline envelop. 

 Cells each with one reddish stigma which is more prominent 

 in the anterior part of the colony. No connecting filaments 

 between the cells; nonsexual reproduction by gonidia which 

 are formed by increase in size of a part of the cells of a colony. 

 Daughters escape from parent as small colonies of bi-flagellate 

 cells which at this stage arc all similar. Se.xual reproduction 

 not known. 



Fig. 151. Pleodorina illinoisensis Kofoid. X 335. (After Kofoid.) 



87 (84, 85, 86) 



Colony spherical, of 8 or 1 6, 32 or 64 cells evenly scattered near 

 the surface of a gelatinous sphere. . . Eudorina Ehrenberg. 



Cells spheriail or oval, with two cilia and a pigment 

 spot. Chromatophore single, parietal. Vegetative re- 

 production .by repeated division, forming at first a 

 plate-like daughter colony, which later becomes spher- 

 ical. Sexual reproduction by a pear-shaped anthero- 

 zoid and a spherical oosphere. 



The cells lie at the surface of the gelatinous sphere 

 and the cilia project at right angles to the surface. All 

 of the vegetative cells may become transformed into 

 oogonia and antheridia; in each of the latter 64 anther- 

 ozoids are formed. The ripe oospores are brownish 

 with a smooth external membrane. The habitats of 

 Eudorina are ponds, ditches, and lakes. 



Fig. 152. £«<forino e/f|an5 Ehrenberg. (.\fter Stein.) 



88 (80, 83) Colony a larger gelatinous sphere with a very large number of 

 minute cells at the surface Volvox Linnaeus. 



Cells very small, round or pear-shaped, connected by protoplasmic filaments, each with a 

 pair of cilia, a single chromatophore and two or more contractile vacuoles; reproduction sexual 

 and asexual; in the latter certain cells (parthenogonidia) within the sphere enlarge and through 

 divisions give rise to a new colony. Sexual reproduction occurs by the union of a fusiform 

 antherozoid and oosphere; oospore spherical, with red contents and a spiny membrane. 



89 (69) Colonies not motile in the vegetative condition 90 



90 (95, 131, 175) Cells in colonies, generally sessile and enclosed in a definite 

 gelatinous envelop, or borne on gelatinous stalks. 

 Reproduction asexual by zoospores, or sexual by 

 isogametes. . . . Family Tetrasporaceae . . 91 



91 (94) Cells biciliate, at the surface of an inflated, attached colony. Cilia 

 external and free 92 



