86 THE MICROSCOPE AND ITS REVELATIONS. 



in the general configuration of the shell, which are indicated by the name 

 it bears. 



480. Globigerinida. Returing once again to the simple ' monothala- 

 mous ' condition, we have in Orbulina a minute spherical shell that 

 presents itself in greater or less abundance in Deep-sea dredgings from 

 almost every region of the globe a globular chamber with porous walls, 

 and a simple circular aperture that is frequently replaced by a number of 

 large pores scattered throughout the wall of the sphere. It is maintained 

 by some that Orbulina is really a detached generative segment of Globi- 

 gerina, with which it is generally found associated. The shell of Globi- 

 gerina consists of an assemblage of nearly spherical chambers (Fig. 325), 

 having coarsely porous walls, and cohering externally into a more or less 

 regular turbinoid spire, each turn of which consists of four chambers 

 progressively increasing in size. These chambers, whose total number 

 seldom exceeds sixteen, do not communicate directly with each other, 

 but open separately into a common ' vestibule ' which occupies the centre 

 of the under side of the spire. This type has recently attracted great 

 attention, from the extraordinary abundance in which it occurs at great 

 depths over large areas of the Ocean-bottom. Thus its minute shells 



Fig. -825. 



Globigerina bulloides, as seen in three positions. 



have been found to constitute no less than 97 per cent of the i ooze 9 

 brought up from depths of from 1260 to 2000 fathoms in the middle of 

 the northern parts of the Atlantic Ocean. The surface-layer of this ooze, 

 the thickness of which is entirely unknown, consists of Globigerins& 

 whose chambers are occupied by the sarcodic bodies of the animals, and 

 which may therefore be presumed to be living on the bottom; whilst its 

 deeper layers are almost entirely composed of dead and disintegrating 

 shells of the same type. The younger shells, consisting of from eight to 

 twelve chambers, are thin and smooth; but the older shells are thicker, 

 their surface is raised into ridges that form a hexagonal areolation 

 round the pores (Fig. 326, A); and this thickening is shown by examina- 

 tion of thin sections of the shell (B) to be produced by an exogenous de- 

 posit around the original chamber-wall (corresponding with the ' inter- 

 mediate skeleton ' of the more complex types), which sometimes contains 

 little flask-shaped cavities filled with sarcode as was first pointed-out by 

 Dr. Wallich. But the sweeping of the upper waters of the Ocean by the 

 ' tow-net ' ( 217), which was systematically carried-on during the voyage 

 of the ' Challenger,' brought into prominence the fact that these waters 

 in all but the coldest seas are inhabited \)j floating Globigerinae, whose 

 shells are beset with multitudes of delicate calcareous spines, which ex- 

 tend themselves radially from the angles at which the ridges meet, to a 

 length equal to four or five times the diameter of the shell (Fig. 327). 

 Among the basis of these spines, the sarcodic substance of the body 



