44 TOILERS IN THE SEA. 



increasing thickness of the walls of the various 

 chambers, from the latest back to the earliest, the 

 increased thickness being due to the successive depo- 

 sition of thin external strata of calcareous matter, 

 which strata would consequently be the most 

 numerous in the oldest chambers, diminishing regu- 

 larly and consecutively towards the most recently- 

 constructed chamber. " Having so spread itself out 

 as an investing body, it appears to have deposited 

 new calcareous layers, covering over the greater part 

 of the pre-existing external segments ; but in each 

 layer so added open points have been left opposite 

 to the mouths of the pseudopodian tubes in the layers 

 previously formed, reminding us of the way in which 

 similar apertures are left opposite the mouths of the 

 canaliculi in membraniform bone-growths. In some 

 instances, however, especially near the umbilical 

 region, these tubes have been blocked up by the 

 more recent investments." 



We might also refer to the shell structure itself, 

 and especially in those species in which " the outer 

 layers are perforated in every direction by a network 

 of minute anastomosing canals, which communicate 

 freely with the exterior of the shell through numer- 

 ous minute apertures." All these minute details 

 of elaboration are marvellous, especially in the 

 construction of the skeleton of animals so small, 

 and of such simple organisation, that they are 



