52 



THE INVERTEBRATA 



the gametes differ in size. They unite indifferently, so that some of 

 the unions are isogamous, though most are anisogamous. The zygote, 

 after a period of encystment, becomes a free flagellate and divides to 

 form a colony. In fresh waters. 



Eudorina (Fig. 3 a). Colonies which differ from those of 

 Pandorina in that: {a) the zooids are spaced on the inside of the 

 common envelope, though connected by strands of protoplasm; 

 {b) the sexual reproduction is strongly anisogamous, since in some 

 colonies the zooids do not divide but, becoming somewhat larger, act 

 as macrogametes, while in others each zooid divides into a bundle of 



Fig. 43. Pandorina. From Godwin, a, The adult colony of sixteen similar 

 flagellated zooids, x 200. h, A colony undergoing asexual reproduction, 

 X 450 — each zooid has divided to form a daughter colony which still re- 

 mains within the parent body. Some of the colonies have already produced 

 flagella, and will shortly break out of the parent cell, b-g, Stages in sexual 

 reproduction — h. Motile gametes, c, Stage immediately after fusion of two 

 gametes, d, Later stage showing flagella withdrawn, e, Later stage showing 

 resting zygote with thickened wall. /, Motile individual produced by the 

 ygote on germination, g, New colony produced by vegetative division of the 

 motile individual. 



16-64 slender individuals (microgametes), which are set free and 

 fertilize the individuals of a macrogamete (female) colony. 



Pleodorina (Fig. 3 b, c). Rather larger colonies which differ from 

 those of Eudorina in that some of the zooids do not perform repro- 

 duction. These zooids, which are smaller than the rest, are termed 

 ''somatic". 



Volvox (Figs. 44-46). Large, subspherical colonies resembling in 

 general features those of Pleodorina but having a much smaller pro- 

 portion of reproductive zooids. Those zooids which perform asexual 



