CHLOROPHYCEA 179 
lives in the tissues of Sarcophyllis, have been insuffi- 
ciently studied, and probably are identical with 
Chlorocystis. In Chlorocystis the zoospores (or gametes) 
leave the mother-cell without any mucilaginous 
envelope. No gametes are known in Halosphara, 
and zoospores are formed after the division of the 
nucleus into numerous nuclei. Round these nuclei 
the protoplasm and chromatophores assemble and 
form plano-convex bodies, each of which divides by 
constriction into two zoospores. These are conical in 
shape, with a slight protoplasmic projection at the 
base, from which the two cilia arise. Characium has 
zoospores showing difference of size, but all with two 
cilia, and these escape by an apical opening of the 
cell. In Sykidion the zoospores are all of the same 
size, but before their formation the cell-contents are 
enclosed within a new membrane. Codiolum has 
zoospores with two cilia, and also motionless spores. 
The Geographical Distribution of marine Proto- 
coccacee is very imperfectly known. All the genera 
occur in British seas, except a doubtful genus, 
Pleurocapsa, recorded from the Adriatic. Nearly all 
the marine forms are known only in European waters, 
except Chlorocystis Sarcophyct, which is Australian. 
The pelagic forms are widely distributed, but too 
little known to permit of a definite statement. 
PLEUROCOCCACEA. 
General Characters—This order, like its allies, 
is more largely represented in fresh-waters than in 
N 2 
