260 Mr. W. H. Leigh-Sharpe on 
careful consideration to the matter of the claims of MNemato- 
cystis and Pleurocystis as independent genera, I have come 
to the conclusion that the characters of my specimens agree 
in the main with those diagnostic of Pleurocystis except in 
regard to a small discrepancy as to size. Hesse states that 
Pleurocystis is 2 mm. long and Nematocystis 5 mm., whereas 
my specimens are of the length of 4-5 mm. If the animal is 
as rare as Hesse implies, it is possible that it may attain 
dimensions greater than that menticned by him, or, again, 
that an English variety may be Jarger than the French one 
which he found in and around Dauplhiné. There exists, as 
far as I can ascertain, no previous record of Pleurocystis 
occurring in the British Isles, though it has possibly been 
found and confused with the presumed conjugation stage ot 
Monocystis magna, which Hesse has now determined is not 
a conjugation but a life-association into diploids, thereby 
deciding him to alter its name to Nematocystis magna. 
The statement of Ceconi (1903) that in Monocystis agilis, 
at any rate, each conjugant separately forms a cyst around 
itself, which afterwards coalesces with that of its partner, 
was scouted by Cuénot. Such is not true at any rate for 
Pleurocystis, where the double (cuticular) separation between 
the individuals is never broken down. The cuticle exhibits a 
network of fine striations, and as the cytoplasm shrinks away 
from it by plasmolysis, it may perhaps have been mistaken 
for acyst. Some of the specimens were ruptured, and others 
easily became so at the slightest touch. ‘The endoplasm was 
plentifully stored with paramylum (or paraglycogen). 
Specimen A, which is also the largest, being 1 mm. longer 
than any other diploid, is markedly differeut from the rest. 
It exhibits a kind of caudal formation, as though it had been 
killed while making Euglenoid movements; further, the 
cytoplasmic contents extend right to the posterior extremity, 
and are more densely granular. Conversely, the other 
specimens present a normal specific outline, the cytoplasm is 
withdrawn from the posterior extremity as though shrinkage 
was occurring previous to cyst-secretion. From these obser- 
vations I infer that A is an earlier, and possibly the earliest, 
stage of diploid association, and that the other figures represent 
subsequent stages. ‘The figures of no other observer represent 
any such caudal formation. 
Though I have figured a nucleus in two individuals, these 
were not apparent through the cytoplasm ; but, upon rupture 
of the cuticle, when the contents become extruded the nucleus 
is plainly visible. The nucleus is situated about hallway 
