ii8 THE FORAMINIFERA 



it generally becomes areolated on the surface, the deposit being 

 greatest between the pores, so that these open into cup-shaped 

 depressions, separated by ridges. In many, but not all pelagic 

 specimens the shell is produced on all sides into radiating cylindri- 

 cal spines which spring from the points where the ridges meet, 

 and may exceed the diameter of the shell in length. 1 



A large proportion of the individuals, which in their earlier 

 stages conform to the type of Globigerina bulloides, complete their 

 growth under this form ; but for others a different future is in 

 store. Having attained a size which may be equal to that of the 

 full-grown test of the other specimens, or may fall considerably 

 short of it, these secrete a large spherical chamber which usually 

 encloses the whole of the previously formed test, and is frequently 

 more than double its diameter (Fig. 49, right-hand cut). The 

 enclosed test is usually only connected with the investing wall at 

 the points where its spines meet the wall and unite with it. 

 The investing chamber is perforated by large pores, with a 

 diameter of from 13-21 /A, as well as by minute pores (5-6 //). 

 The specimens which form the spherical chamber have been 

 given the generic name Orbulina. It will be convenient to use the 

 terms Orbulina chamber, and Globigerina chambers for the invest- 

 ing and the invested chambers respectively. 



Unlike the Foraminifera which creep over the sea-bottom, the 

 pelagic Globigerinae may be found invested with a vacuolated 

 covering which is in part gelatinous (38, p. 6), though traversed 

 by radiating pseudopodia which project beyond it. This en- 

 velops the whole shell and the bases of the spines, and has a 

 spherical contour. It is probable that the Orbulina chamber is 

 secreted at the surface of this vacuolated mass. A similar cover- 

 ing may be found investing the Orbulina shell in the later phases 

 of the life-history. 



As in the free Globigerina, the outer surface of the Orbulina 

 chamber is beset with spines, which vary greatly in length, and 

 specimens have been found, though rarely, the surface of which is 

 areolated by ridges as in Globigerina. These have been separated 

 under a distinct name 0. porosa, Terquem. 



Rhumbler finds that, in pelagic specimens, the Globigerina 

 chambers are always present within the Orbulina shell, though, 



1 Sir John Murray thus describes the appearance of the living animal : " In 

 Globigerina bulloides (hirsuta) and aequilateralis the yellow -orange colour of the 

 sarcode is due to the presence of numerous oval-shaped xanthidiae or ' yellow cells,' 

 similar to those found in the Radiolaria. When the sarcode with these ' yellow 

 cells ' flows out of the foramina, and mounts between the numerous spines outside 

 the shell, the whole presents a very striking object under the microscope ; the trans- 

 parent sarcode can be seen running up and down the long silk-like spines, and the 

 'yellow cells ' seated at the base of these spines quite obscure the body of the shell." 

 Nat. Science, July 1897, p. 20. 



