444 Natural History of British Zoophytes. 
crystallization of calcareous matter excreted by the living polypife- 
rcus bark, and once excreted, beyond their power to change it, ex- 
cepting by the addition of material of the same quality. And to 
confirm this inference, anatomy lends its aid, for no structure of 
vessels or of cells in which fluids could circulate, can be detected 
in the axis ; nor can any vessel be shown which can maintain a 
connection between it and the living pulp. 
There are certain discrepancies which might render it doubtful 
how far the same theory can be safely applied to explain the for- 
mation of the polypidoms of the Hydraforrn and Ascidian polypes : 
the axis of the corticiferous species is internal, the polypidoms of 
the latter are always external ; the former is solid, the latter are 
tubular sheaths or cells, and occupied with the polypes, or with the 
living pulp, of which the polypes are merely a developement. But 
the new position and form of these polypidoms has not altered their 
real nature, for, as in the preceding, their texture is homogeneous, 
like horn or shell, in which no vascular or cellular structure, simi- 
lar to that of organs growing by imbibition and assimilation, can be 
detected, nor can they be made to evince a sign of the lowest ir- 
ritability by the application of any stimulus. And further, be- 
tween the polypes, and their sheath or cells, there is no organical 
connection, and even no very close apposition in the hydracolous 
tribe, so that when the polype moves in its cell whether to retreat 
within its cavity or expand itself beyond the rim the cell suffers 
no change or alteration in shape or capacity ; it remains, under all 
the mutations of the lively and protsean inmate, invariably the same. 
Seeing, then, that they are undoubtedly extravascular, the ne- 
cessary inference seems to be, that the polypidoms in question can- 
not grow as a plant grows, but must be evolved from and moulded 
on a living basis, and can be no other than an exudation from the 
surface of the living flesh : and, accordingly, it has been ascertain- 
ed by experiment, that if this fleshy substance is destroyed, no horny 
sheath or skeleton is formed. In the reproductive gemmule there 
are two substances, viz. the pulp and the thin cuticle or membrane, 
the latter of which is the germ of the future arborescent or cellular 
polypidom : by the growth of the pulp the membrane is distended 
and moulded into a cell, or pushed upwards in the form of a shoot, 
in which, after a time, the pulp is arrested in its growth longitu- 
dinally, and swells out, and is developed into an animated polype, 
furnished with tentacula, and with a mouth and digestive organs. 
Bursting the cell at the point which becomes the future aperture, 
