80O MICROSCOPIC FORMS OF ANIMAL LIFE 



power, as is shown in figs. 632, 633. 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 

 whose tubuli are occupied either by air or by any substance having 

 a refractive power different from that of the intertubular substance, 

 however perfect may be the transparence of the latter. The straight- 

 ness, parallelism, and isolation of these tubuli are well seen in verti- 

 cal sections of the thick shells of the largest examples of the group, 

 such as Nummulites (fig. 631). It often happens, however, that 

 certain parts of the shell are left imchannelled 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. 



Between the comparatively coarse perforations which are common 

 in the rotaline type, and the minute tubuli which are characteristic 

 of the nummuline, there is such a continuous gradation as indicates 

 that their mode of formation, and probably their uses, are essen- 

 tially 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. 607) ; 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 approximation 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 per- 

 forated rotalines. Hence it is obvious that the tubulation or non- 

 tubulation of foraminiferal 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 medium, and contribute by their action to the nutrition 

 of the segment from which they proceed, these pseudopodia are 

 limited in the latter case to the final segment, issuing forth only 

 through the aperture of the last chamber, so that all the nutrient 

 material which they draw in must be first received into the last seg- 

 ment, and be transmitted thence from one segment to another until 

 it reaches the earliest. With this difference in the physiological con- 

 dition of the animal of these two types is usually associated a further 

 very important difference in the conformation of the shell viz. 



