110 



expanding its newly-formed segment to such a degree as to cover 

 the organism with its thin gelatinous tissue. It must be borne in 

 mind that such an investment would only require a retral expansion 

 of the animal ; the V-like form of each segment would enable it, in 

 its normal state, to cover an area extending from one umbilical 

 region to another, equal to that of its own diameter. 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, espe- 

 cially near the umbilical region, as seen in fig. 2, these tubes have 

 been blocked up by the more recent investments. 



After thus covering over at least a considerable part of the exte- 

 rior of the shell with new lamellae, the soft animal appears to have 

 retreated to the limited area which the new segment was ultimately 

 destined to occupy ; and here the new lamellse, continuously pro- 

 longed, instead of remaining in close contact with the pre-existing 

 shell, have sprung up from its surface, in order to form a calcareous 

 covering for the newly-formed segment, which would, in its turn, 

 be invested and rendered denser by numerous repetitions of the 

 same process. I can see no other way of explaining the growth of 

 the smooth and glossy shells of Amphistegina gibbosa. The proba- 

 bility of its correctness is somewhat enhanced by an important fact, 

 for the knowledge of which I am indebted to Professor E. Forbes. 

 He remarks in a letter, " When dredging in the Shetlands six years 

 ago, I took some good Nodosarw alive ; and after carefully watching 

 them for a day, I saw the substance of one of them extended from 

 the orifice in the form of moniliform strings of granular substance." 

 This observation, made by such a distinguished naturalist, establishes 

 the fact that these organisms can extend themselves beyond the 

 limits of their calcareous cells, through the narrow apertures usually 

 termed oral, and consequently makes it more probable that the 

 soft animal of Amphistegina had the power of extending itself in a 

 similar way ; reminding us of the ordinary movements of the difflu- 

 gian Infusoria. Supposing the above explanation to be correct, it 

 still remains a question for investigation whether the entire animal 

 has been able to leave its cell, or whether the expansion has only 

 occurred to the budding segment prior to its having been enclosed 



