SOILS AND MOSSES OF SPITSBERGEN. 459 
cyst is well figured by Puschkarew (27), but the two walls are always separable, 
and the granule and vacuole are sometimes poorly developed or even absent. 
Though these two species seem quite distinct, apparently intermediate 
forms both of the active and encysted stages are frequently found, the identi- 
fication of which is very difficult, and in cultures of either species a few 
abnormally large or small individuals are usually found which might be 
mistaken for members of the other species. Until conclusive observations in 
pure cultures have been carried out, there consequently remains a doubt 
whether we are actually dealing with two true species or with two extreme 
forQis of a single very variable species. 
Sainoukon mikkotbron* (n. gen., n. sp.). (Pi. 24. figs. 1-6.) 
A brief description of this organism under the name of Sp. 7 has already 
been published (13). It is a small flagellate from 3/t to 6/i in length with 
a single flagelhim, slightly longer than the body, which arises just behind the 
anterior end and is directed backwards. The anterior end is rounded, and 
usually the body is slightly crescent-shaped (fig. 2) (the side from which the 
flagelluQi arises being slightly concave) and tajiers somewhat towards the 
posterior end. In these specimens the maximum width is about half the 
length or less, but sometimes individuals occur in which ihe posterior end is 
swollen, and the side from which the flagellum arises is sigmoid in outline, 
the concavity being limited to the anterior end. In normal cultures the 
shape is quite persistent, but in the deeper parts of hanging-drop cultures 
plastic individuals sometimes occur adhering to the glass slides (figs. 4, 5). 
The plastic individuals contain ingested bacteria, which are absent in the 
normal free-swimming form, the nutrition of the latter being apparently 
wholly saprophytic. The nucleus is not visible in the living animal, probably 
owing to its small size, and no contractile vacuole has ever been observed. 
The movements are very like those of a small Heteromita, since the 
flagellum is trailed quite passively while the animal progresses at a con- 
siderable speed with a rapid "\-ibratory motion, the body being turned at an 
angle of about 20° to the direction of motion. The progression is in a 
straight line (not spiral), and the animal does not turn on its axis as do most 
organisms which possess an anterior flagellum. 
When stained, a large vesicular nucleus becomes visible in the anterior 
part. The basal granule varies considerably in size and position, at times 
lying against the anterior side of the nuclear membrane and at other times 
being quite free from it and apparently not connected to it by any rhizo- 
plast. Most characteristic of this species, however, is a large mass of 
plastin, often visible in the living animal as a refringent body, which lies 
just behind the nucleus or at times forms a ring completely enclosing it 
(figs. 1-3). In a dividing animal this body divides immediately after 
* araivoi, to was'. 
