4 BRITISH FRESHWATER RHIZOPODA. 
extraneous materials the genus Difflugia may be taken 
as the type. In this case the substances used are 
selected by the animal from among its surroundings ; 
some species choose only fine grains of sand of nearly 
equal size, others grains of various dimensions, others 
again prefer diatom-frustules exclusively or mixed 
with quartz-particles; it is very seldom that vegetable 
débris is utilized although one or two species (e.g. 
D. leidyi) normally incorporate a few pieces of vege- 
table origin in the structure of their tests. 
Some species construct their tests either partially 
or almost entirely from scales or plates which have 
formed’ tests belonging to other species or genera, 
and it seems not improbable that such tests are dis- 
integrated for the, purpose, as isolated scales from 
decayed tests are far from common, at any rate are 
rarely found in gatherings of mud or sand. 
The uniformity of choice of extraneous matter by 
each species indicates that the proper material, so far 
as possible, is chosen instinctively, but at the same 
time a search and a choice has to be made among 
the mud, sand, and débris amid which the animal 
exists. 
We sometimes find that although the body of the 
test is fairly uniform in size, shape, and materials, the 
spines, horns, and projections may vary considerably, 
within, of course, certain limits, either in form, number, 
or position; thus, among the Huglyphe, whilst the type 
may have a moderate number of spines on its test, 
varieties occur nearly or quite glabrous, or on the 
other hand provided very numerously with spines; 
again, among the Difilugix, the number of the horns on 
cornuted tests often varies considerably; thus D. con- 
stricta may be glabrous or provided with from one to 
six horns on its test. 
More stable and seldom showing so much variation 
are the modifications of the tests themselves, such as 
the internal pockets and depressions characterising the 
tests of such species as Nebela equicaleeus, N. martial, 
