Natural History of British Zoophytes. 
237 
same type, have made considerable advances towards complexity of 
organization, and their relation to the polypidom is entirely alter ^ 
ed. Hitherto the polypidom has been, what its name imports, a 
cell for retreat in danger, and in ordinary an extravasular insensi- 
ble sheath to protect the contained animal from the rude contact of 
the circumfluent element ; but now we find it occupying an inter- 
nal position, and instead of a covering it has become a sort of prop 
or skeleton to a fleshy crust in which the polypes are immersed. 
In the Alcyonium this interior support is scarcely to be recognized 
in some calcareous spicula scattered through the central mass, but 
in Pennatula it forms a bone stretched like a vertebral column from 
one extremity to the other, and in Gorgonia it is ramified into 
branches after the manner of a tree. It is this axis, under what- 
ever shape it appears, which is the true analogue of the polypidom 
of the ascidian and hydraform polypes, although the name certainly 
has no suitableness here, for the polypes cannot only not nestle in 
that which is uncellular, but they have no immediate connection 
with it. They, as already mentioned, are found lodged in a sort of 
cell (Fig. 4. a.) exca- Fig. 4. 
vated in an exterior sar- 
coid crust, which con- 
stitutes the main bulk 
of the polypiferous 
mass, and which, in 
fact, is nothing more 
than a production of 
the bases and outer 
skin of the polypes 
hardened by a deposi- 
tion of calcareous gra- 
nules and spicula, and 
made more coriaceous 
in texture, to bear with 
impunity the contact 
and ruffling of the wa- 
ter. * This crust is 
accordingly a living 
irritable structure, permeated by tubes prolonged from the po- 
* " Lorsqu'on observe les Alcyons dans leur etat naturel, la ligiie de demarca- 
tion entre ces deux parties parait bien tranche, et on pouvrait au premier abord, 
croire ces petits animaux loges dans des cellules an pourtour de I'ouverture des- 
quelles ils adhereraient ; mais quand on eleve a 1'aide d'un acide utendu d'otiu, 
a. 
