72 THE MICROSCOPE AND ITS REVELATIONS. 



The surface of the shell, which has a peculiarly ' porcellanous ' aspect, is 

 marked by closely-set strice that cross the spaces between the successive 

 septal bands; these markings, however, do not indicate internal divisions, 

 and are due to a surface-furrowing of the shelly walls of the chambers. 

 This type passes into two very curious modifications; one having a spire 

 which, instead of flattening itself out, remains turgid like that of a 

 Nautilus, having only a single aperture, which sends out fissured exten- 

 sions that subdivide like the branches of a tree, suggesting the name of 

 Dendritina ; the other having its spire continued in a rectilineal direc- 

 tion, so that the shell takes the form of a crosier, this being distin- 

 guished by the name of Spirolina. A careful examination of inter- 

 mediate forms, however, has made it evident that these modifications, 

 though ranked as of generic value byM. D'Orbigny, are merely varietal; 

 a continuous gradation being found to exist from the elongated septal 

 plane of Peneroplis, with its single row of isolated pores, to the arrow- 

 shaped, oval, or even circular septal plane of Dendritina, with all its 

 pores fused together (so to speak) into one dendritic aperture; and a like 

 gradation being presented between the ordinary and the 'spiroline' 

 forms into which both Peneroplis and Dendritina tend to elongate 

 themselves. 



464. From the ordinary nautiloid multilocular spiral, we now pass to 

 a more complex and highly-developed form, which is restricted to 

 tropical regions, but is there very abundant that, namely, which has 

 received the designation Orbiculina (Plate xv., figs. 6, 7, 8). The 

 relation of this to the preceding will be best understood by an examina- 

 tion of its earlier stage of growth, represented in fig. 7; for here we see 

 that the shell resembles that of Peneroplis in its general form, but that 

 its principal chambers are divided by 'secondary septa' passing at right 

 angles to the primary, into ' chamberlets ' occupied by sub-segments of 

 the sarcode-body. Each of these secondary septa is perforated by an 

 aperture, so that a continuous gallery is formed, through which (as in 

 Fig. 316) there passes a stolon that unites together all the sub-segments 

 of each row. The chamberlets of successive rows alternate with one 

 another in position; and the pores of the principal septa are so disposed, 

 that each chamberlet of any row normally communicates with two 

 chamberlets in each of the adjacent rows. The later turns of the spire 

 very commonly grow completely over the earlier, and thus the central 

 portion or l umbilicus ' comes to be protuberant, whilst the growing edge 

 is thin. The spire also opens out at its'growing margin, which tends to 

 encircle the first-formed portion, and thus gives rise to the peculiar shape 

 represented in fig. 8, which is the common aduncal type of this organ- 

 ism. But sometimes, even at an early age, the growing margin extends 

 so far round on each side, that its two extremities meet on the opposite 

 side of the original spire, which is thus completely inclosed by it; and 

 its subsequent growth is no longer spiral but cyclical, a succession of 

 concentric rings being added, one around the other, as shown in fig. 6. 

 This change is extremely curious, as demonstrating the intimate relation- 

 ship between the spiral and the cyclical plans of growth, which at first 

 sight appear essentially distinct. In all but the youngest examples of 

 Orbiculina, the septal plane presents more than a single row of pores, 

 the number of rows increasing in the thickest specimens to six or eight. 

 This increase is associated with a change in the form of the sub-segments 

 of sarcode from little blocks to columns, and with a greater complexity 

 in the general arrangement, such as will be more fully described here- 



