DIFFLUGIA CONSTRICTA. A438 
var. ecornis (Ehrenb.) Leidy, are often similar to the 
accepted type of D. constricta. 
In both species the test is chitinous and more or 
less covered with extraneous particles; if these consist 
of closely-set grains of sand or silicious plates entirely 
hiding the chitinous membrane and giving a grey 
colour to the test, it is identified with D. constricta ; 
when however the adherent matter is more or less 
scattered, leaving parts of the chitinvus membrane 
exposed, and the testis of a brown or yellowish colour, 
it is usually identified with (entropyxis aculeata. 
Unfortunately living occupants of tests are not 
Fic. 172.—Difflugia constricta var. spinifera. x 320. (After Cash.) 
common, «nd active individuals are very rarely seen 
under the microscope, so that frequent comparison of 
the pseudopodia and movements of the various forms 
cannot be made, and the opacity of the tests prevents 
observation of the living plasma and its inclusions. 
Observers such as Wallich, Leidy, Penard, and Cash, 
amongst others, have all expressed their inability to 
clearly differentiate certain forms of the two species, 
which it would certainly appear advisable to include 
under one name, indicating the various forms as 
varieties or forme. 
Tests occur from which may be formed a graduated 
series varying in shape from Heliw-like to Arcella-like, 
the former usually conforming to the D. constricta type, 
the latter to C. aculeata ; spines may or may not be 
present on any of them, but are more usual and more 
numerous on the latter. 
