THE FORAMINIFERA. 



79 



be its form, is either perforated or imperforate. In the 7m- 

 perforata ( Gromidce, Lituitidw, Miliolidce) the pseudopodia 

 are protruded from only one end of the body, the rest of 

 which is cut off from the exterior by the skeleton. In the 

 Perforata the substance of the shell is traversed by more or 

 less delicate canals filled with the protoplasm, which thus 



Fig. 3.— Diagrams of Foraminifera. — J., monothalamian ; B, C, polythalamian ; D, 

 horizontal ; i<7and F, vertical sections of helicoid form. In F, the chambers of 

 each turn of the spiral overlap their predecessors and conceal them, as in the 

 genus Nummulites. 



reaches the surface and gives off pseudopodia all over the 

 body. Hence, while the hard parts of the Imperforate/, form 

 a sort of exoskeleton, those of the Perforata have rather 

 the nature of an endoskeleton. 



The simplest skeletons are spherical or flask-shaped, and 

 single-chambered. But complication arises by the addition 

 of new chambers, which may form a linear series, or be coiled 

 upon one another in 1 various ways, or be irregularly aggre- 

 gated. Moreover, the new chambers may overlap those al- 

 ready formed indifferent degrees, and the interspaces between 

 the walls of the chambers may be variously filled up by sec- 

 ondary deposition until such large and apparently compli- 

 cated bodies as the Nummulites are built up. 



The Foraminifera are almost all marine animals, living 

 in the sea, from the surface to great depths, sometimes free, 

 and sometimes attached to other bodies. 



The investigations of Major Owen, confirmed and extend- 

 ed by the recent work of H. M. S. Challenger, have proved 

 that such forms as Globigerina^ Pulvinuleiria, and Orbulina, 



