GENERAL STRUCTURE OF POLYCYSTINA. 449 



a much larger proportion of Foraminiferous shells than of the 

 accompanying particles ; so that, a large portion of the extra- 

 neous matters being thus got rid of, the final selection he- 

 comes comparatively easy. Certain forms of Foraminifera are 

 found attached to shells, especially bivalves (such as the Cha- 

 macece), with foliated surfaces ; and an extensive examination of 

 those of the Indian Seas, when brought home "in the rough," 

 has yielded to Mr. W. K. Parker some most valuable and novel 

 results, which will be made public in due time. 



293. The final selection of specimens for mounting, should 

 always be made under some appropriate form of Single Micro- 

 scope ( 27-30) ; a fine camel-hair pencil, with the point wetted 

 between the lips, being the instrument which may be most con- 

 veniently and safely employed, even for the most delicate speci- 

 mens. In mounting Foraminifera as microscopic objects, the 

 method to be adopted must entirely depend upon whether they 

 are to be viewed by transmitted or by reflected light. In the 

 former case, they should be mounted in Canada balsam ; the 

 various precautions to prevent the retention of air bubbles which 

 have been already described ( 128), being carefully observed. 

 In the latter, no plan is so simple, easy, and effectual, as the 

 attaching them with a little gum to a blackened disk of card, 

 and guarding them by a perforated wooden slide ( 123). They 

 should be fixed in various positions, so as to present all the dif- 

 ferent aspects of the shell, particular care being taken that its 

 mouth is clearly displayed ; and where, as will often happen, the 

 several individuals differ considerably from one another, special 

 care should be taken to form series illustrative of their range of 

 variation, and of the mutual connections of even the most di- 

 verse forms. For the display of the internal structure of Fora- 

 minifera, it will often be necessary to make extremely thin sec- 

 tions, in the manner already described ( 108-110) ; and much 

 time will be saved, by attaching a number of specimens to the 

 glass at once, and by grinding them down together. For the 

 preparation of sections, however, of the extreme thinness that 

 is often required, those which have been thus reduced should 

 be transferred to separate glasses, and finished oif each one by 

 itself. 



294. Polycystina. It is probable that we are to refer to the same 

 general type, an extensive group of very interesting microscopic 

 bodies, possessing great beauty and variety of form and structure 

 (Figs. 210-216) of whose essential character extremely little is 

 known. These are minute siliceous shells, which appear, from 

 the recent observations of Prof. Muller, to contain in the living 

 state an olive-brown " sarcode," extending itself into pseudo- 

 podial prolongations (resembling those of Actinophrys, 262), 

 that pass through the large apertures by which the shells are 

 perforated. The sarcode body does not seem always to fill the 

 shell ; being stated by Prof. Muller to occupy, in the Encyrtidium 



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