4 . BRITISH FRESHWATER RHIZOPODA. 



extraneous materials the genus Dijjlugia 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 

 debris is utilized although one or two species (e. g. 

 D.-leidij'i) 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 debris 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 Euglyplise, 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 Difflugise, 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 equicalceus, N. martiali> 



