es, 
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CoNSTRUCTION oF tHE TESTS OF THE CONCHULINA. 
Owing to the wide range of variation in the form 
of tests which occurs in many species of testaceous 
Rhizopoda, the determination of the limits of species 
and varieties is a subject fraught with difficulty and one 
upon which there are many diverse opinions. A brief 
resumé of the conditions, so far as we know them, 
under which these tests are constructed, would seem 
not out of place here and should be helpful in con- 
sidering the problems so frequently presented to 
observers of Rhizopod life. 
There are broadly two classes of tests, those secreted 
by the animal itself and those which it builds up of 
extraneous materials; the former are less subject to 
variation than the latter, not of course taking into 
account those variations of shape in flexible tests 
which the animal can execute at will or which are due 
to ingested food, and this greater conformity to type 
is only what might be expected to occur in the case of 
those tests which are formed of natural secretions and 
are moulded on the animal’s body. There are forms 
of secreted tests in which whilst the elements of the 
test are secreted naturally in the plasma, the test as 
a whole is built up from these elements by the efforts 
of the animal itself ; thus, in the genus Huylypha, when 
a new danghter test is about to be produced, all the 
necessary elements may be seen accumulated in readi- 
ness in the plasma; aperture-scales, body-scales, spine- 
scales, or loose spines can all be distinguished among 
the collection, but their sorting-out, building-up, and 
cementing together are dependent upon the animal’s 
more or less voluntary efforts, and when the appa- 
rently very inadequate equipment of the animal for 
this purpose is considered, it is surprising that the 
tests should exhibit so much uniformity. 
Of those Rhizopoda which form their tests from 
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