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tion derived ? We can comprehend each segment secreting, and 

 surrounding itself with, a layer of calcareous matter ; but how is the 

 exterior of this nucleus reached ? In the Testacea such a point might 

 be accomplished by a voluntary extension of its mantle by the mol- 

 lusk. In the present instance, 1 would suggest as possible, seeing that 

 we have no such mantle capable of extension, that the central nucleus 

 of lime may have been thickened, through the instrumentality of the 

 pseudopodia which appear to penetrate it from the inner convolutions, 

 and which, if this explanation prove to be correct, will thus possess an 

 additional function to those already noticed. This, however, is a mere 

 suggestion, arising from the difficulty of accounting for the appearance. 

 The only other explanation which appears plausible, is, that the 

 newly-forming segment, before enclosing itself in its calcareous cell, 

 extends itself over the central nucleus, and then, gradually receding 

 to what constitutes its permanent limits, leaves a calcareous layer be- 

 hind it. The former idea however appears to me the most plausible 

 one. De Montfort, in suggesting the existence of an extensile man- 

 tle capable of polishing the outer surface, had doubtless seen the dif- 

 ficulty of comprehending how the internal animal was to reach the 

 exterior of the shell. The structure above described is most evident 

 in the beautifully polished specimens which abound in the Mediter- 

 ranean, and with which he was doubtless well acquainted. The true 

 use of the rows of marginal processes (figs. 6, « , and 7, c) it is not 

 easy to determine. The fact that each of their truncated extremities 

 appears to come into direct contact with the foraminated wall bound- 

 ing the preceding cell — the septum at that point seemingly consisting 

 of a single calcareous layer, instead of a double one, as is the case 

 with it elsewhere — rather indicates the probability of some indirect 

 communication being kept up, through their means, between conti- 

 guous cells. This might be accomplished through the ordinary fo- 

 ramina already described, without necessitating any direct continuity 

 or attachment of the one to the other ; it is scarcely to be supposed 

 that they have not some other nature than that of being mere saccu- 

 lated appendages to the general digestive cavity ; this, however, is a 

 question for further investigation. I have seen nothing resembling 

 either ova within the chambers, or the supposed petiolated egg-sacs 

 of Ehrenberg attached to the exterior of Polystomella crispa. Ova 

 of some kind must exist ; and in the interior of some forms of Rota- 

 lia and Rosalina I occasionally find small bodies amazingly like 

 spores, but I have found none in any species of Polystomella. 



The new cells appear to be most probably added by gemmiparous 



