THE MASTIGOPHORA 177 



particles being carried down on the outer surface of the collar, at 

 the base of which they sink into the body of the cell. 



Several of the genera are found both in the sea and in fresh 

 water. 



There are two orders of Choanoflagellata : 



ORDER 1. Craspedomonadina, Stein. 

 A. NUDA, Lankester. 



Monosiga, Kent ; Diplosiga, Frenzel (with two collars, one within the 

 other) ; Hirmidium, Perty ; Codosiga, Kent (Fig. 7 (3, 4)). 



B. LORICATA, Lankester. 



Salpingoeca, Clark (Fig. 7 (1, 6, 7)); Polyoeca, Kent; Sphaeroeca, 

 Lauterborn. 



ORDER 2. Phalansteriina ( = Gelatinigera, Lankester). 



"The cell-units secrete a copious gelatinous investment and form 

 large colonies." 



Phalansterium, Cienkowski (Fig. 5 (12)), with inconspicuous collars; 

 Proterospongia, Kent (Fig. 7 (15)), with conspicuous collars. 



SUB-CLASS III. PHYTOFLAGELLATA. 



The Phytoflagellata or Volvocaceae are clearly related to the 

 Chromomonadidea, and some authors include this order in the sub- 

 class. Now that it has been definitely ascertained that conjugation 

 does occur in many of the Euflagellata, the formation of a zygote by 

 the copulation of two gametes is a feature that does not distinguish 

 the Phytoflagellata from the other sub-classes of the Mastigophora. 

 Moreover, although in Copi'omonas and some other Monadidea the 

 conjugating individuals cannot be distinguished from the asexual 

 individuals, definite micro- and megagametes are formed in the life- 

 history of Mastigella, Trypanosoma, and others. The phenomenon of 

 gametogenesis therefore is not a distinguishing character of the 

 sub-class. The Phytoflagellata, however, exhibit a much more 

 definite approximation to a purely vegetable structure than any 

 of the Euflagellata, and it may be convenient to keep them together 

 for the present in a separate sub-class. 



The sub-class includes solitary and colonial forms, and the body 

 of the cell-unit is enclosed by a firm cell-wall which sometimes takes 

 the form of a bivalvate shell (Phacolus). In the colonial forms the 

 cell -units are embedded in a gelatinous matrix. There is no 

 indication of pharynx, nutrition being holophytic except in the case 

 of Polytoma, which is a colourless, saprophytic Chlamydomonad. 



12 



