4 Mr. II. J. Carter on the Alteration 



until the wliolc of tlie shell had hecome transformed into the 

 chitinous skeleton of a llydr actinia, when, probably finding 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 clianged into the peculiar growth of the llydr actinia, 

 which presents a more or less irregularly tubercled aj)])earance, 

 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 elathratc fibre involving one or more of the smooth 

 erect s])iiies which cliaracterizc the species (fig. 1, a, d)\ whereas 

 in I lydractinia echinata tiiere are no such tubercles, the sur- 

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



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

 spectively of the transformation ; 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 (whicli was made for the purpose, and forms part of 

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

 and 2-1 2ths in the parietes. Moreover these layers show, by 

 the ])resence of smooth S})incs u))on them here and there (fig. 2, 

 dd), that the growth of the Ilijdractinia 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 

 tlu; Ihidnictinia which lined its cavity (fig. 2, e e), since in the 

 s])e(;imen of llydractinia echinata before me the polype-crust, 

 although smoothed by the P««7?^r«*s internally , covers the cavity 

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

 the transformation, 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 transfornuition inside it for what may have been effected 

 by the lower ])art 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 llydractinia echinata ; and thus it is satis- 

 factorily proved that this kind of poly})e can effect a change in 

 the composition of a shell analogous to that produced by the 

 sponges mentioned. 



This is ;i ])oint of interest to know, inasmuch as it bears on 

 fossilized as well as recent structure, and therefore every 

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

 hastily set down as sponge-transformation. 



