46 THE INVERTEBRATA 



and in particular among the Volvocina there are interesting cases 

 intermediate between hologamy and merogamy, and between isogamy 

 and anisogamy. Thus in Polytoma the age at which the products of 

 fission unite varies in a species, so that some are merogametes while 

 others, delaying, become hologametes ; in Pandorina (p. 58) isogamy 

 and anisogamy are facultative ; and various species of Chlamydomonas 

 (see p. 31) make up a series in which there is a transition from com- 

 plete isogamy to a pronounced anisogamy which rises to oogamy in 

 Volvox and other colonial forms. 



The zygote is very commonly encysted. 



The Mastigophora fall into a number of fairly well-defined orders. 

 It is convenient to group these by their nutrition into two subclasses 

 — the Phytomastigina, containing orders most of whose members are 

 holophytic (see p. 18), and the Zoomastigtna, which have no holo- 

 phytic members — but all the orders of the Phytomastigina contain 

 some colourless members, whose nutrition is purely saprophytic, 

 and all except the Volvocina include colourless holozoic forms. 

 Owing to this fact it is impossible to frame a definition which will 

 enable every member of each subclass to be recognized as such with- 

 out comparison with other species. Certain characteristics, however, 

 distinguish most members of the Zoomastigina from most of the 

 colourless Phytomastigina. These characteristics are stated below, in 

 the section which deals with the Zoomastigina. 



Subclass PHYTOMASTIGINA 



Mastigophora which possess chromatophores, and species without 

 chromatophores which closely resemble such forms. 



There can be no doubt, for reasons which have been given above, 

 that this subclass contains the most primitive members of the phylum. 

 Its nutrition is extraordinarily interesting from that point of view. 

 Some of its species, notably among the Volvocina, are purely holo- 

 phytic. Others are normally also saprophytic, and some of these, like 

 Euglena, can upon occasion practise this mode of nutrition alone. Yet 

 others, like Polytoma, have become colourless, and are purely sapro- 

 phytic. Others again are both holophytic and, by amoeboid in- 

 gestion, holozoic. These lead insensibly to similar forms, members of 

 the Zoomastigina (Monas, etc.), which, being without chromato- 

 phores, have not the faculty of photosynthesis, but are purely animal 

 in their nutrition. Some of the coloured forms which possess a pit 

 that is called a gullet are said to take food with it, and thus to combine 

 holophytic and holozoic nutrition. In any case certain of their relatives 

 which have lost the chromatophores {Cyathomonas, Peranema, etc.) 

 take solid food through a similar gullet. Most of the holozoic forms 



