6 Mr. H. J, Carter on the Alteration 



pidom of Hydractinia echinata is formed of horizontal layers 

 (figs. 8 & 9), each of which is marked by a row of knots 

 (e, figs. 8 & 9), which indicate the points of union of the 

 clathrate chitinous fibre, corresponding to the knots in network ; 

 and, judging from a microscopic examination of the part ad- 

 vancing into the shell, it would also seem that these knots first 

 appear in the form of separate cells (fig. l^dd), which, gene- 

 rating concentric layers of chitine around them, may be termed 

 " horn-cells." The horn-cell then sends off two sets of 

 branches, one of which (fig. 7, e e) becomes the clathrate chi- 

 tinous fibre, which is solid and formed of concentric layers, and 

 the other set (fig. 7, ff) spread out into a chitinous membrane 

 (fig. 7, g) on the same plane as the horn-cells, which membrane 

 thus acts as a framework to the whole. These horn-cells 

 appear as dark points in the last layer of shell-substance that 

 is about to be absorbed, and which remains adherent to the 

 contracted and curled-up fragments of the dried and thus 

 broken-up poly2)idom, as above mentioned (fig. 5, a ; fig. 8, g). 

 The chitinous membrane therefore lies above this (fig. 6, h ; 

 fig. 8,y). But if a fragment of these two layers, viz, the chi- 

 tinous and calcareous ones (which are of course very thin, but 

 can be occasionally picked off" together), be mounted in Canada 

 balsam, it will be observed that the calcareous layer, which is 

 the undermost, presents a worm-eaten appearance (fig. 7, «'), as 

 if it had been subjected to the dissolving influence of a sm-face 

 formed of pseudopodial villi, about 1 -6000th inch in diameter. 



In the layer lining the cavity of the wholly transformed shell 

 (fig. 2, e e), treated in a similar manner, we have the same cha- 

 racters, minus, of course, the calcareous layer, as in fig. 9, g^ — 

 that is to say, the chitinous membrane alone, in which are set 

 the horn-cells and their clathrate structure, as in a, figs. 6 & 7. 



How the absorption of the shell-substance is efiected in 

 Hydractinia is unknown to me ; but (referring to like phe- 

 nomena) when we observe that the protoplasm of the plant-cell 

 can, as required, work its way through the thick cellulose cell 

 (as in Bpirogyra under conjugation), that the tender Amoeba- 

 like entophyte Pythium (also an inhabitant of the cell of 

 Spirogyra) will do the same thing, &c., that the excavating 

 sponges, whose sarcode is equally soft and delicate, will do the 

 same in the oyster-shell as well as in limestone rock, it does 

 not appear strange that the coenosarc of Hydractinia should be 

 able to perforate a Avhelk-shell under similar circumstances. 



Also, when it is observed that, in the excavations made by 

 Cliona celata in the concretionary limestone formed and found 

 about the rocks of the New Red Sandstone on the shore here, 

 the siliceous grains which are mixed up with it still project 



