THE FORAMINIFERA. 



79 



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

 perforata ( Gromidce, Jyltuitldae^ Miliolidm) 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.— Diairrams of Foraminifera.—A, monothalamian ; B, C, polythalamian ; /?, 

 horizouial ; A' and I^\ vertical sectious of helicoid form. In ^, ihe clianibers of 

 each turn of the spiral overlap their predeceissors and conceal them, aa in the 



genus NimiinuUtes. 



reaches the surface and gives off pseudopodia all over the 

 body. Hence, while the hard parts of the Imperforata 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 various ways, or be irregularly aggre- 

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

 ready formed in different 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 Foramhufera are almost all marine animal?, 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 Globlr/erlna, Pulvinularla, and Orbidina, 



