12 THE VOYAGE OF H.M.S. CHALLENGER. 



is produced by pullulation from the circumference of the first-formed nucleus} 

 which occupies the centre of the disk ; and this " nucleus " consists of a " primordial 

 segment " a, from one end of which is given off a larger " circumambient seg- 

 ment " h, which passes completely round it, and is itself surrounded by the first 

 aunulus. The shelly disk (fig. 2) which encloses this sarcodic body, and is (so to 

 speak) modelled upon it, is marked on each surface by a series of distinct concentric 

 circles, the spaces between which are channeled-out in the interior into concentric series 

 of chamberlets, connected together by annular galleries ; and the cavitary space of each 

 zone is connected with that of the next by short radial passages, of which one usually 



Fio. 2.— Tyjiical jilan of structure of slielly Disk of OrbitulUes. 



a, Primordial chamber. 



h. Circumambient chamber; together forming a "nucleus," which is surrounded by concentric rings of chamberlets 

 connected with each other by annular galleries and radial passages, the latter appearing as pores along the margin. 



passes-ofi" from one of the short galleries that connect the chamberlets of each zone, into 

 a chamberlet of the zone that surrounds it. These passages, in the outermost zone, open 

 as " pores " on the margin of the disk ; these orifices constituting the only means of com- 

 munication between the cavitary system of the disk and the outer world. Each concen- 

 tric zone, when itself the outermost, thus communicated directly with the exterior ; but 

 each, when surrounded by another zone, can only do so through its intermediation, 

 what were in the first place its marginal pores, being closed-in by a new annulus of shell, 

 and opening into its chamberlets. The " nucleus " of the shell, round which its first 

 annulus is formed, contains a " primordial chamber " (fig. 2, a), surrounded by a " circum- 

 ambient chamber " h ; and, in the highest or most specialised representatives of the Orbito- 

 line type, radial passages {e, e, e, fig. 3) are given-off" from the whole circumference of this 

 " cu'cumambient " chamber, which carry stolon-processes (PI. V. fig. 18) that swell into 

 the sarcodic sub-segments which occupy the successive annular series of chamberlets e, c, c. 



^ This use of a term which has an altogether different and well-understood signification in Biology, is doubtless 

 open to objection ; and I can only plead in excuse that having employed it in my original Memoir, published when 

 that signification was far more limited, I have not been now able to think of any other which should be equally 

 applicable. The term centrum might have been substituted, if it were not that (as I shall hereafter show) the " nucleus " 

 is often cxcentric. 



