THE PORIFERA AND PROTOZOA 



321 



than the diameter of the chamber) in the last-formed 

 chamber, and (in one sub-order) through numerous 

 perforations of the whole surface. Moreover, the fora- 

 minifer does not cut off new chambers by secreting 

 septa each " septum " formed the terminal part of the 

 outer shell before it was converted into an internal 

 structure by the building of a new chamber in front of it. 

 The ontogeny reveals in many cases a striking dimorphism : 

 in the same species growth may begin by a very 



FIG. 96. DIMORPHISM IN A FORAMINIFER, BILOCULINA 

 BULLOIDES, D'ORBIGNY, RECENT. 



a. Exterior showing aperture with T-shaped partition. (X^--) b t Cross- 

 section of microspheric and c, of megalospheric form. ( X 125.) Only 

 the early chambers are shown, up to the attainment of the regular 

 alternation of chambers. Chambers numbered in order of develop- 

 ment. (After Schlumberger. ) 



small chamber (microspheric form) or by a much larger 

 one (megalospheric form). The study of living forms 

 has shown that this is due to an alternation of genera- 

 tions. 



The microspheric form is produced by a sexual pro- 

 cess, and its ontogeny is complete ; the megalospheric is 

 produced asexually, and the early stages are skipped 

 altogether, just as they are in a rejuvenated coral (Fig. 

 96, b, c). The microspheric form in some species attains 



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