Mineral Deposit in the Rhizopods and Sponges. 



77 



bei- — a" being the principal aperture, through which the sarcode- 

 mass is seen to bulge outwards. 



Fig. 5. 



The difference between these two chambers deserves special 

 attention. As already stated, the first layer of shell is deposited 

 from the immediate surface of the sarcode-mass within, being 

 only interrupted at the main aperture and those points through 

 which the stolons and pseudopodia make their escape. In the 

 normal condition of the organism, no further deposit takes place 

 within. Every subsequent addition to the thickness of the shell- 

 wall is made from without, and is brought about by a special 

 layer of sarcode which spreads over the entire external surface 

 from the stolons*, and thus seems to secrete the shell-substance 

 by its gradual retrocession outwards, as in the case of the wall 

 of the cavity in which the sponge-spicule is formed. This outer 

 layer of sarcode may very readily be seen in mature Globigerina, 

 and it is probably present in all Foraminifera, although visible 

 with difficulty in some, too subtle to be appreciable in others, 

 and perhaps taking its origin, in the imperforate genera, by a 

 distinct reflexion of the sarcode- substance through the main 



* These stolons, unlike those which take part in the secretion of silex 

 in the case of the sponge-spicule, seem designed actually to prevent the 

 deposit of calcareous matter wherever they occur ; since otherwise no 

 apertures would remain for communication between the internal sarcode 

 of the chambers and the medium in which the organism lives. 



