Structure of Tests of Fresh-water JRhkopoda. Ill 



The examination of these structures entails careful manipula- 

 tions, being opaque in air or water ; immersion in oil of cloves or 

 balsam, which renders them translucent, is necessary before the 

 internal structure is distinguishable, and then close scrutiny from 

 every point of view and careful lighting are requisite. 



The curved vermiform rods forming the test of Lesquereusia 

 spiralis Biitschli, and similar ones often incorporated in the test of 

 Nebela griseola Penard (8), may be formed from diatom frustules, 

 but they are so reduced in diameter and bent into all manner of 

 curves that we can only conjecture their origin in many cases 

 (PL III, fig. 6). 



The tests in Group 3 comprise those which are formed, either 

 wholly or in part, of extraneous materials modified by the animal ; 

 these materials are usually silicious, such as sand-grains, frustules 

 of diatoms, and sometimes the scales or plates of other tests, 

 although these are usually employed without modification. The 

 walls of the test when completed resemble a mosaic, the tessera 

 fitting closely together and adjusted so that no large intervals are 

 left ; even between those of most varied forms they are cemented 

 together with a chitinous cement. The walls are of equal thickness 

 or made thicker in those parts where extra strength is required, and 

 are finished off to a smooth surface both inside and out. 



Of the tests made partially as above, some species of Hdeopera 

 provide good examples ; H. petricola Leidy, a common species, has 

 a test made of sand-grains which are smoothed off around the 

 aperture and anterior part of the test, but near the crown larger 

 grains usually project and the surface is rough ; the test of H. nodosa 

 Wailes (9) shows this tendency carried to an extreme, the apex 

 consisting of a nodular mass of grains of quartz (PI. II, fig. 5) 

 which must, one would think, greatly impede free locomotion. The 

 cementing medium is highly resistant to the action of acids, 

 and probably consists of chitin mixed with very fine particles of 

 silica. There is also a general tendency among most species of this 

 genus for the tests to assume an amethystine tint which is very 

 brilliant in such species as H. rosea Penard and H. petricola var. 

 ametlufstea Penard ; the shades vary from pale violet to a rose-red, 

 but the colours usually seen are similar to those produced by oxide 

 of manganese ; the colouring matter is probably confined to the 

 cementing material with which the quartz-grains are perhaps coated'. 

 In the case of Awerinzewia cyclostoma (Pen.) Schouteden, this is 

 evidently the case, as the test is usually composed of rather large 

 grains of a chalky white tint embedded in a violet-coloured matrix. 



The genus Nebela contains many abundant and widely dis- 

 tributed species with tests composed of modified extraneous 

 materials, and of plates or disks which appear to be secreted by 

 the animal itself, or obtained from some other test; sometimes it 

 is possible to recognize the exact species from which some of the 



