of Shell-suhstance hy Hydractinia. 3 



granular plasma called "coenosarc;" and the external parts of 

 the skeleton serve to support the polype-mass ; all of which, 

 being extremely delicate, fail, after being once dried, to present 

 under any circumstances a recognizable form. 



If we now, with a very sharp and thin knife, cut off verti- 

 cally a slice of the di-ied curled-up crust above mentioned (PI. I. 

 fig. 8) and place it under a microscope, we may see the white 

 crystalline shell-substance {d d) gradually decreasing upwards 

 among the interstices of the chitinous network, until it gives 

 place entirely to the dark amber-colom* of the latter and its 

 granular plasma (c, figs. 8 & 9) ; while in the opposite direction 

 the white substance increases to the confines of the lowermost 

 layer of the network, until it can hardly be distinguished from 

 the substance of the shell itself (fig. 8,^'). 



Again, if we put a similar slice (fig. 9) into dilute nitric 

 acid, we shall not only observe an effervescence, but when 

 this slice is placed under the microscope we may also observe 

 that the whole of the white crystalline substance has dis- 

 appeared (fig. 9, c?^), leaving nothing but the clathrate fibre 

 of the skeleton (fig. 9, e), of which the increasing thinness, 

 pale colour, and wide interstices towards the shell evince its 

 young or progressive stage of development. 



It thus appears evident that Hydractinia ecMaata trans- 

 forms the calcareous shell on which it may be growing into 

 its own horn-like skeleton. 



We have now to prove this more satisfactorily ; and this can 

 be done by another specimen in the British Museum, where 

 the whole of the shell has become transformed into the horn- 

 like skeleton of a Hydractinia. 



The shell thus transformed was somewhat less in size than 

 a Buccinunij but of a totally different family, as may be seen 

 by the form of the aperture, which resembles that of some tur- 

 binated shell, though of course the species is now undeter- 

 minable, at least to one possessing such a limited knowledge 

 of conchology as myself (fig. 1, a, h). Nor is the Hydractinia 

 the same specifically as H. echinata ; for all the spines are 

 smooth (fig. 3), and not, as in the latter, serrated (fig. 4). 

 Hence there is here a marked difference between the two 

 polypes, although in every other respect the skeleton-mass or 

 polypidom, which is the only part left in the transformed shell, 

 is almost identical with that of Hydractinia echinata. 



As the transformed shell now exists, it is empty and entirely 

 composed of parallel layers of clathrate chitinous fibre (fig. 2, 

 o, h). The internal cavity is smooth, and the columella pre- 

 served ; so that we may fairly infer that the shell had been 

 originally tenanted by a Pagurus^ which had remained there 



1* 



