4 Mr. H. J. Carter on the Alteration 



until the whole of the shell had become transformed into the 

 chitinous skeleton of a Hydractinia, when, probably linding it 

 too light for its purpose, the Pagurus betook itself to a heavier 

 habitation. 



Although the internal surface of the transformed shell 

 remains smooth and perfect (fig. 2, a, &), the external surface 

 has become changed into the peculiar growth of the Hydractinia, 

 which presents a more or less irregularly tubercled appearance, 

 each tubercle of which, being more or less separate from the 

 rest and varying in size and shape, consists of a little monti- 

 cule of clathrate fibre involving one or more of the smooth 

 erect spines which characterize the species (fig. 1, a, c?); whereas 

 in Hydractinia echinata there are no such tubercles, the sur- 

 face being for the most part even and equally spined throughout. 



So much, then, for the internal and external surfaces re- 

 spectively of the transfonnation ; we have now to go to the 

 layers of which it is composed. And these together present a 

 thickness varying with that of the original shell, being in the 

 section (which was made for the purpose, and forms part of 

 the illustrations, fig. 2) 4-12ths of an inch thick at the base, 

 and 2-12ths in the parietes. Moreover these layers show, by 

 the presence of smooth spines upon them here and there (fig. 2, 

 dd), that the gi'owth of the Hydractinia had been outwards as 

 well as inwards or towards the shell. Nor does it seem quite 

 clear how much of the shell has been absorbed by the layer of 

 the Hydractinia which lined its cavity (fig. 2, e e), since in the 

 specimen of Hydractinia echinata before me the polype-crust, 

 although smoothed by the Pagurus internally, covers the cavity 

 as well as the exterior of the shell. At the same time, in 

 the transfoi-mation, the presence and direction of the spines 

 on its layers (fig. 2, c c) point out, to a certain extent, the limit 

 of the crust vertically, leaving about one third of the thickness 

 of the transformation inside it for what may have been effected 

 by the lower part of the outer crust and that lining the cavity 

 respectively. In this case the original shell could not have 

 been very thick. 



A microscopic examination of the structure gives the same 

 results, minus the soft substance and presence of calcareous 

 matter, as that of Hydractinia echinata ; and thus it is satis- 

 factorily proved that this kind of polype can effect a change in 

 the composition of a shell analogous to that produced by the 

 sponges mentioned. 



This is a point of interest to know, inasmuch as it bears on 

 fossilized as well as recent structure, and therefore eveiy 

 clathrate structure of this kind in a fossil shell must not be too 

 hastily set down as sponge-transformation. 



