KHIZOPODA. 133 



difficulties attending the study of the Rhizopoda, and insists very pro- 

 perly upon the necessity of viewing them in all positions, and under 

 different modes of illumination and of preparation, in order to arrive 

 at a due conception of their conformation. 



When recent Foraminifera are dissolved in dilute acid, an organic 

 basis is always left after the removal of the calcareous matter, accurately 

 retaining the form of the shell, with all its openings and pores. The 

 earthy constituent is mainly carbonate of lime ; but Dr. Schultze has 

 satisfied himself of the presence of a minute amount of phosphate of 

 lime in the shells of recent Orbiculina adunca from the Antilles, and 

 of Polystomella strigttata from the Adriatic. 



The solitary Rhizopoda, furnished with a horny shell or capsule, 

 forming a case for the animal, constitute the family Arcellidce. In the 

 genus Arcella, from which the family derives its name, the shell is some- 

 what of a bell-shape, with a very large round opening. In Englypha 

 it is of an oval or flask-like form, with the opening at the smaller end, 

 and the shell appears as though formed of a sort of mosaic of small 

 horny pieces. In Difflugia the shell is often globular. 



All the Polythcdamw, QY Foraminifera, inhabit the sea, and fre- 



fig. 93. 



1. Separated prisms from outer layer of Pinna shell. 



2. Skeletons of Foraminifera from limestone. 



3. Eecent shell of Polystomella. crispa ; seen by the aid of the dark-ground. 

 * illuminator. 



