RHIZOPODA. 17 



The terms porccllanous and vitreous have been adopted owing to the appear- 

 ance of the shells as seen under the microscope. The former is applied to shells 

 of a white, opaque, often shiny appearance, which in thin, transparent sections or 

 laminae appear, by transmitted light, of a brown or amber color. No structure can 

 be observed in shells of this kind. They are never perforated, although they are 

 sometimes marked upon the surface ^vith pits, or inequalities, giving an appearance 

 of foramina. 



The vitreous or hyaline shell-structure is far more complex than the porccl- 

 lanous. It is transparent, usually colorless, sometimes deeply colored, and more or 

 less closely perforated either with large or small distinct foramina, or minute tubuli 

 passing directly through the shell-substance. In JRotalia, Fig. 13, the foramina are 

 distinct, and afford passages for the sarcode, which covers the outside of the sheU, 

 and the pseudopodia extending in all dh-ections fi-om it. The minutely tubular 

 structure can only be detected in thin sections with high powers of the microscope, 

 when it imparts a peculiar appearance to the shell, characteristic of finely tulmlar 

 structures. 



Between the shells with large foramina and with minutely tubular structure, there 

 is a continuous gradation, which 

 indicates that both foramina and 

 tubuli subserve the same pur- 

 pose, — affording channels for 

 the passage of the sarcode. 

 Comparing the shells of the 

 porcellanous and vitreous forms, 

 it will be seen that while the 

 pseudopodia of the animals oc- 

 cupying the former all spring 

 from the teiTuinal or outer 

 chambers alone, so that the 

 nourishment for the sarcode 

 of the inner chambers must 

 pass in through those that in- 

 tervene, in the yitreous forms 

 the sarcode of each chamber is 



in direct communication with the outer world through either the foramina or the 

 minute tubuli of the shell. In accordance with this difference in the structure of 

 the shell-substance, it may be also observed that the stolons of sarcode connecting 

 successive chambers of the porcellanous-sheUed species are much larger than those in 

 the vitreous-shelled forms. 



These facts may be best illustrated by comparing two of the most highly-developed 



! forms of the two tj-pes of shell-structure. For this purpose we will select Orbitolites 

 of the porcellanous, and Nummvlina among the tubular-shelled forms. 



The structure of Orbitolites can be understood by a alance at Fig. 1.5. Disks such 

 as are here represented sometimes attain the size of a silver quarter-doDar in diameter. 

 It will be seen that single pores unite successive chambers, and finally the sarcode of 



|i the outer chambers communicates with the surrounding medium by pseudopodia pro- 



J jected through the marginal pores shomi in the figure. 



j In N^ummulina, a form that has been so abundant in the past as to have lent its 



y VOL. I. —2 



