58 THE INVERTEBRATA 



zooid to form a group of 16 like the parent colony, the dissolution of 

 the colonial and zooid envelopes, and the setting free of 16 young 

 colonies ; (2) sexually, by the division of each zooid and the setting 

 free of its products as gametes which, except in size, resemble ordinary 

 zooids. Since the number of fissions in the formation of gametes 

 differs in different colonies, the gametes differ in size. They unite in- 

 differently, 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 en- 

 velope, 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 macro- 

 gametes, while in others each zooid divides into a bundle of 16-64 

 slender individuals (microgametes), which are set free and fertilize the 

 individuals of a macrogamete (female) colony. 



Pleodorina (Fig. 36, 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 with smaller and more 

 numerous zooids, of which a much smaller proportion is reproductive. 

 Those zooids which perform asexual reproduction are known as 

 parthenogonidia : the plates of young zooids which arise by their radial 

 fission, curving into spheres to form the new colonies, bulge into the 

 hollow of the parent colony, where they remain for a time before they 

 are set free. The clusters (antheridia) of microgametes arise in the 

 same way. In some species the microgametes are considerably modi- 

 fied, being pale, very slender, and bearing their flagella in the middle 

 of their length. Male, female, and asexual reproductive zooids may 

 be found in any combination in a colony. Details of the structure of 

 the colonies are shown in Figs. 45, 46. 



Subclass ZOOMASTIGINA 



Mastigophora which do not possess chromatophores and are not 

 otherwise practically identical with coloured forms. 



By one or more of the following peculiarities of the Zoomastigina 

 most members of the group are distinguished from most colourless 

 members of the Phytomastigina. 



I. The Zoomastigina never have starch or other amyloid re- 

 serves. 



