GENUS POLTSTOMELLA : CANAL-SYSTEM. 547 



in a radial direction (Plate XVII. fig. 2, d, d', Plate XVIII. fig. I,/ 1 ,/ 1 ), from the meridional 

 canals of each convolution to the stolons which unite the segments of that convolution. 

 These, which may be distinguished as the converging branches, are, however, much less 

 regular in their distribution than those which pass outwards from the meridional canals 

 to the stolons of the succeeding whorl. 



191. Thus, then, it becomes apparent that by means of the two spiral canals, the 

 number of convolutions of which equals that of the whorls of the shell, the very 

 numerous meridional canals, of which there is one for every segment of each whorl, 

 the vast multiplication of pairs of diverging branches, of which each meridional canal 

 sends off a number equal to that of the connecting stolons between the segments, and 

 the very considerable aggregation of converging branches, which probably do not fall far 

 short of the preceding except in being single whilst they are in pairs, a very complete 

 system of intercommunications is maintained between the external surface and even the 

 innermost portions of the shell. That these passages are occupied, in the living animal, 

 by prolongations of the sarcode-body, there can scarcely, I think, be any reasonable 

 doubt ; and when we look to the remarkable development of what has been elsewhere 

 termed the " intermediate skeleton," but which may here be more appropriately termed 

 the " supplemental skeleton," namely, the secondary calcareous deposit which not only 

 forms the solid nucleus, but spreads itself over the entire surface, adding considerably 

 to the thickness of the spiral lamina, it cannot be deemed improbable that the special 

 purpose of the canal-system is the formation and nutrition of this supplemental skeleton, 

 which has obviously no direct relation to the segments of the animal body contained 

 within the chambers. Through the trumpet-shaped diverging branches which open in 

 such numbers upon the surface of those chambers, and the straight canals which arise 

 from the nucleus, there will be abundant opportunity for the sarcode-body to extend 

 itself over the whole exterior of the shell, and thus to form any additional deposit upon 

 its surface. 



192. Having carefully re-examined P. crispa with a view of ascertaining to what extent 

 the canal-system is developed in it, I am enabled to state that I have found it to possess 

 a canal-system distributed on the same plan with that of the organism we have been 

 considering, but much more limited in its extent, as might be expected from the much 

 inferior development of the " supplemental skeleton." In the vertical section repre- 

 sented in Plate XVII. fig. 9, we see the solid umbilical protuberances a, a', traversed by 

 straight canals, which terminate in the pits on their surface that were originally noticed 

 by Professor WILLIAMSON, and supposed by him to give exit to pseudopodia. At b, b, b', b', 

 we see the transverse sections of the spiral canals, from which the straight canals just 

 mentioned may be presumed to proceed ; and at c c, dc 1 , we see plain indications of a 

 system of interseptal canals, resembling those of Operculina in their aspect and distri- 

 bution, but clearly homologous, in their relations to the spiral canals, with the meri- 

 dional canals of P. craticulata. I have not been able to detect any indication that 

 diverging branches are given off from them, to pass through the spiral lamina to the 



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