RHIZOPODA. 



17 



The terms porcellanous 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 

 lamiaee 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 with pits, or inequalities, giving an appearance 

 of foramina. 



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

 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 Rotalia, Fig. 13, the foramina are 

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

 and the pseudopodia extending in all directions from it. The minutely tubular 

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

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

 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 terminal 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 vitreous 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-shelled species are much larger than those ia 

 the vitreous-shelled forms. 



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

 forms of the two types of shell-structure. For this purpose we will select OrUtolites 

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



The structure of OrUtolites can be understood by a glance at Fig. 15. Disks such 

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

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

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

 jected through the marginal pores shown in the figure. 



In Nummulina, a form that has been so abundant in the past as to have lent its 

 VOL. I. — 2 



Fig. 15. 



• OrUtolites, enlarged. Diagram to show external appear- 

 ance and internal structure. 



