8o6 MICROSCOPIC FORMS OF ANIMAL LIFE 



light through them ; but in those which are too opaque to be thus 

 seen through, it is sufficient to rub down one of the surfaces upon a 

 stone, and then to mount the specimen in balsam. Each of the 

 superficial elevations will then be found to be the roof or cover of an 

 ovate cavity or * chamberlet,' which communicates by means of a 

 lateral passage with the chamberlet on either side of it in the same 

 ring ; so that each circular zone of chamberlets might be described 

 as a continuous annular passage dilated into cavities at intervals. 

 On the other hand, each zone communicates with the zones that are 

 internal and external to it by means of passages in a radiating 

 direction; these passages run, however, not from the chamberlets of 

 the inner zone to those of the outer, but from the- connecting pas- 

 sages of the former to the chamberlets of the latter ; so that the 

 chamberlets of each zone alternate in position with those of the zones 



FIG. 609. Orbitolites. Ideal representation of a disc of complex type. 



internal and external to it. The radial passages from the outermost 

 annulus make their way at once to the margin, where they termi- 

 nate, forming the ' pores ' which (as already mentioned) are to be seen 

 on its exterior. The central nucleus, when rendered sufficiently 

 transparent by the means just adverted to, is found to consist of a 

 ' primordial chamber ' (a), usually somewhat pear-shaped, that com- 

 municates by a narrow passage with a much larger ' circumambient 

 chamber ' (b), which nearly surrounds it, and which sends off a vari- 

 able number of radiating passages towards the chamberlets of the first 

 zone, which forms a complete ring round the circumambient chamber. 1 



1 Although the above may be considered the typical form of the Orbitolite, yet, 

 in a very large proportion of specimens, the first few zones are not complete circles, 

 the early growth having taken place from one side only; and there is a very beautiful 

 variety in which this one-sidedness of increase imparts a distinctly spiral character 

 to the early growth, which soon, however, gives place to the cyclical. In the Orbiio- 

 litesitalica (fig. 611), brought up from depths of 1,500 fathoms or more, the ' nucleus ' 



