72 BRITISH FRESHWATER RHIZOPODA. 
produce a slight jerky motion of the body from side 
to side. 
The nucleus is prominently situated, appearing in 
some individuals as an irregularly-rounded body, 
strongly refractive, and about 9 win diameter. Schulze 
remarks that it is surrounded by a clear space, and 
that within its substance may be detected “a number 
of minute globular, sharply-bounded clear spots, which 
have the power of altering their positions.” 
Usually two contractile vacuoles may be observed 
imbedded in the densely-granular protoplasm near 
the periphery, on each side. Their pulsations appear 
to be very languid. 
The bacilliform spicules, of which mention has been 
made, line the outer surface of the body in a thin 
stratum, apparently imbedded in a mucous film. They 
vary in number; in some individuals so few as to be 
hardly perceptible ; in others thick enough to justify 
the specific name given to the organism. 
In water from a pond at Northen Etchells, Cheshire, 
about the end of June, a form of Mastigamalba occurred 
which presented a remarkable variation from the type. 
The posterior extremity, instead of being broadly 
rounded, formed a circular expansion of ectoplasm, 
finely granular, containing some small vacuoles, and 
fringed with short radiating conical or acute pseudo- 
podia, of very delicate structure. A few minute 
spicules could be detected on the surface of this 
appendage, but neither on the body nor on the pseudo- 
podia proper were any visible. The pseudopodia, in 
these abnormal examples, were more attenuated and 
fewer in number than in the others, and at the same 
time more pellucid. The flagellum was active in most 
cases, seeming to perform the function of a tentacle. 
The individuals were comparatively small, and generally 
lighter m colour, and they had a wider margin of 
ectoplasm. For the present we retain this (PI. VL f. +) 
as a variety—M. aspera, var. cestricnsis. 
