a 
220 THE MEDALS OF CREATION. Cuap. VII. 
is more characteristic of vegetable than of animal structures, 
we proceed to consider the fossil sponges and allied forms, 
as the mineralized remains of the lowest types of the animal 
kingdom : if the vegetable nature of the originals were gene- 
- 
rally admitted, this section should have followed that which 
treats of the Diatomacez (ante, p. 100). 
Sponge consists of a reticulated fibrous mass, covered with — 
numerous pores of various sizes, which are connected inter- — 
nally by anastomosing channels, and this tissue is surrounded 
by a cellular gelatinous matter, by which the entire structure — 
was secreted, and is, in fact, the vital part of the zoophyte. 
The tough framework or skeleton is in some kinds fibrous, 
horny, flexible, or rigid, and strengthened by calcareous or 
siliceous spicula (spines) ;* while in other species its substance 
is calcareous, and in some siliceous, constituting a web of 
transparent rock crystal, resembling spun glass.t The gela- 
tinous matter lines all the cavities, and forms the margins of 
the openings ; it presents no signs of irritability, and may be 
easily pressed out of the porous mass with the hand, so 
slight is the connexion between the skeleton and the in- 
vesting tissue. Currents of water constantly enter the small 
pores, traverse the inosculating canals, and issue from the 
larger orifices, which often project above the surface in per- 
forated papille. By the circulation of the water through 
the porous structure, the nutrition of the organized mass is 
effected ; and the modifications observable in the number, 
size, form, and disposition of the pores, channels, and orifices, 
in different species, appear to be subservient to this especial 
* The Mediterranean and American sponges of commerce are devoid 
of spicules, and are deprived of their soft animal matter simply by 
washing freely in fresh water. 
+ I particularly allude to a siliceous Sponge from Barbadoes, named, — 
by Mr. Samuel Stutchbury, formerly of the Bristol Institution, (now of 
Australia,) Dictyochalix pumicea. This specimen is of a fungiform 
shape, and appears to the naked eye as if formed of pumice stone, but 
under the microscope is literally a tissue of transparent silex. 
. 
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> A OP, 
