TUBULAR SHELL-STRUCTURE. 487 



guished as punctations on the surface of the Shell with a low 

 magnifying power, as is shown in Fig. 248 ; whilst in other cases 

 they are so minute as only to be discernible in thin sections seen 

 by transmitted light under a higher magnifying power, as is 

 shown in Figs. 258, 259. When they are very numerous and / 

 closely set, the Shell derives from their presence that kind of / 

 opacity which is characteristic of all minutely- tubular textures,/ 

 when their tubuli are occupied either by air or by any substancg 

 having a refractive power different from that of the intertubular 

 substance, however perfect may be the transparence of the latter. 

 The straightness, parallelism, and isolation of these Tubuli are 

 well seen in vertical sections of the thick shells of the largest 

 examples of the group, such as Xummulina (Fig. 253). It often 

 happens, however, that certain parts of the Shell are left unchan- 

 nelled by these tubuli ; and such are readily distinguished, even 

 under a low magnifying power, by the readiness with which they 

 allow transmitted light to pass through them, and by the peculiar 

 vitreous lustre they exhibit when light is thrown obliquely on their 

 surface. In Shells formed upon this type, we frequently find that 

 the surface presents either bands or spots which are so distinguished ; 

 the non-tubular bands usually marking the position of the septa, 

 and being sometimes raised into ridges, though in other instances 

 they are either level or somewhat depressed ; whilst the non- 

 tubular spots may occur on any part of the surface, and are most 

 commonly raised into tubercles, which sometimes attain a size 

 and number that give a very distinctive aspect to the shells that 

 bear them. 



370. Now between the comparatively coarse perforations which 

 are common in the Rotaline type, and the minute tubuli which are 

 characteristic of the Nummidine, there is such a continuous gra- 

 dation as indicates that their mode of formation, and probably 

 their uses, are essentially the same. In the former it has been 

 demonstrated by actual observation that they allow the passage of 

 pseudopodial extensions of the sarcode-body through every part of 

 the external wall of the chambers occupied by it (Fig. 248); and 

 there is nothing to oppose the idea that they answer the same 

 purpose in the latter, since, minute as they are, their diameter is 

 not too small to enable them to be traversed by the finest of the, 

 threads into which the branching pseudopodia of Foraminifera are 

 known to subdivide themselves. Moreover, the close approxima- 

 tion of the tubuli in the most finely-perforated Nummulines, makes 

 their collective area fully equal to that of the larger but more' 

 scattered pores of the most coarsely-perforated Rotalines. Hence 

 it is obvious that the tubulation or non -tabulation of Foramini- 

 ferous Shells is the key to a very important Physiological difference 

 between the Animal inhabitants of the two kinds respectively; 

 for whilst every segment of the Sarcode-body in the former case 

 gives off Pseudopodia, which pass at once into the surrounding 



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