68 i 
and the Flagellata. 
longitudinally, and one half becomes the new apical cell while 
the other is displaced below it. 
For reproduction the cells become free and swarm as tetra¬ 
hedral zoospores with a colourless ciliate apex (< c ). These 
subsequently come to rest, attach themselves by the clear 
apex and secrete a long mucilaginous column so as to push 
themselves up on a wide stalk (a). The base of the cell is 
thus directed to the apex of the stalk, and this relation is 
maintained all through the branching system. Lateral 
branches arise acropetally by peripheral cells, at intervals, 
taking on apical growth. Of course there is no sexual repro¬ 
duction. The cells contain no pyrenoids and the product of 
assimilation is a highly refracting liquid, the so-called ‘leucosin.’ 
The essential point which distinguishes Hydrurus from other 
colonial Flagellates is that the whole organism is non-motile 
in the main part of its existence. It only reverts to its ancestral 
motility at the swarm-spore stage, and this of course for bio¬ 
logical advantage in spreading the species. Now this non¬ 
motility is characteristic of the plant-type, and those who 
attach absolute importance to this consider that Hydrurus has 
evolved over the border-line and must be considered to be an 
Alga, though it is clearly on a little branch-phylum of its own 
and not on the main Phaeophyceae-phylum. Its remarkable 
morphological differentiation supports this. If, however, 
motility is to be the absolute test it is clear that conversely 
all the Volvocaceae must be regarded as Flagellates ; Biitschli, 
Scherffel (’ 00 ). Adopting a consensus of characteristics as our 
standard, we may keep the Volvocaceae as plants and yet 
hesitate to call Hydrurus one. 
Phaeocystis. Lagerheim (’ 96 ), Scherffel (’ 98 ), (’00). The 
two known species of this genus form mucilaginous spherical 
colonies visible to the naked eye and floating motionless in the 
sea. The centre of the colony consists of thin mucilage, and 
the naked round cells with their two phaeoplasts are sparsely 
scattered over the denser periphery of this very fragile organism. 
Contractile vacuoles, eye-spots and pyrenoids are absent, and 
leucosin is the product of assimilation. Phaeocystis was first 
