40 THE INVERTEBRATA 



organism we can as yet envisage was multinuclear and photosynthetic, 

 and gave rise independently to the Metazoa and, by reduction of the 

 body, to flagellates, and so to the Protozoa, which on this view are 

 not truly members of the Animal Kingdom. 



Class MASTIGOPHORA (FLAGELLATA) 



Protozoa which in the principal phase possess one or more flagella; 

 may be amoeboid, but are usually pelliculate or cuticulate; are often 

 parasitic but rarely intracellular; have no meganucleus; and do not 

 form very large numbers of spores after syngamy. 



The reproduction of the Mastigophora is in most cases by equal 

 longitudinal fission. The way in which, in many of the solitary 

 Volvocina, this becomes transverse has been described above (p. 24). 

 In the Dinoflagellata fission is oblique or transverse. The fission may 

 be simply binary or repeated. The number of fissions often varies in 

 the same species, and is usually greater in the formation of gametes 

 than in asexual reproduction. Binary fission in forms which have not 

 a stout cuticle usually occurs in the free-swimming stage, but may 

 take place in a cyst or jelly case, as, for instance, occasionally in 

 Eugleiia viridis. In forms with a stout cuticle, as in the Volvocina, the 

 protoplasm shrinks from the cuticle, which serves as a cyst. Repeated 

 fission usually occurs in a cyst. The fate of the flagella at fission has 

 been dealt with on p. 24. The mitoses (see p. 19) in this group range 

 from beautiful eumitoses to the extremest cryptomitoses, the latter 

 generally in parasitic forms. Paramitosis occurs in the Dinoflagellata. 



In many genera syngamy is not known to occur. Among those in 

 which it does, all degrees of diflference between gametes are found, 

 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. 51) isogamy 

 and anisogamy are facultative ; and various species of Chlamydomonas 

 (see p. 26) 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. 14), and the Zoomastigina, 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. 



