2 Mr. H. J. Carter on the Alteration 



once a Gasteropoclous shell, although not a particle of the cal- 

 careous matter may remain, and the whole be transformed into 

 sponge-structure. 



The same thing, mutatis mutandis^ may take place with the 

 flexible polype called Hydr actinia^ which for the most part 

 also forms a parasitic crust on univalve shells. 



Thus in the British Museum there is a specimen of 

 Hydractinia echinata covering a whelk-shell {Buccinum un- 

 datum) both inside and out ; and the same was tenanted by a 

 Pagurus^ now dead in situ ; while the horny skeleton or in- 

 crustation of the polype, having shrunk by contraction on 

 drying, has become cracked about the lip, and the pieces so 

 curled up that their edges have become exposed, and thus show 

 that, although the outer part presents its natural dark amber- 

 colour, the inner one becomes gradually whiter, until it appears 

 to differ very little from the shell itself. 



Carrying on our examination with a simple lens, we observe 

 that the pieces have brought away with them a portion of the 

 shell-substance on which the crust grew ; and when both their 

 lower side and the corresponding sm-face of the shell are re- 

 spectively examined, it will be foimd that the former presents 

 a surface of whitish crystalline matter punctated by amber- 

 coloured points, which are connected aljove with the horny 

 structure of the Hydractinia^ while the surface of the shell 

 opposite presents nothing of the kind, and is therefore uni- 

 formly white, — thus showing that the horny or chitinous in- 

 crustation has brought away with it just so much of the shell- 

 substance as the horny portion of the poly[)e had penetrated. 



Hydractinia echinata is so common on our coast that it does 

 not seem necessary for me to describe here more than the part 

 immediately connected with our subject, viz. the polypidom, 

 which includes the transformation of the substance of the shell 

 into the horny structure of the Hydractinia. For the rest I 

 refer the reader to the ample descriptions, illustrations, &c. 

 contained in Mr. Hincks's ' History of the British Hydroid 

 Zoophytes,' vol. i. p. 19 &c., and vol. ii. pi. 4 (1868). 



The skeleton or polypidom of Hydractinia consists of a 

 clathrate mass of horny solid fibre (I use the word " homy " 

 here synonymously with " chitinous," as the most expressive 

 term, although chemically not so correct as the latter), which 

 spreads horizontally in a thin layer over the shell on which the 

 polype may be growing, rising above into a forest of pyramidal 

 serrated spines, averaging about one sixteenth of an inch high, 

 and descending below by simple advancement of the clathrate 

 fibre into the shell-substance, as before mentioned. 



The insterstices of the clathrate network are filled by the 



