PIFFLUGIA CONSTRICTA. 43 



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 I), constrwta ; 

 when however the adherent matter is more or less 

 scattered, leaving parts of the chitinous membrane 

 exposed, and the test is of a brown or yellowish colour, 

 it is usually identified with Centropyxis actileata. 



Unfortunately living occupants of tests are not 



FIG. 172. Difflugia constricta var. spinifera. x 320. (After Cash.) 



common, and 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 formae. 



Tests occur from which may be formed a graduated 

 series varying in shape from Helix-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. 



