0 eh die 
FOSSIL SPONGES. 991 
purpose ; the imbibition and expulsion of water being the 
_ only function the sponges perform. In its earliest stage the 
_ sponge gemmule is of a spheroidal shape, and covered with 
_ vibratile cilia, and after expulsion from the canals in which 
_ it is formed, moves rapidly through the water till it becomes 
_ attached to some body, and is then immovably fixed during 
life; exhibiting no signs of vitality save the aqueous circula- 
_ tion through the pores and canals. 
There is much confusion in the arrangement and nomen- 
clature of the fossil species of this class of zoophytes ; and 
this has originated, in part, from the varied forms assumed 
by the same species, having been described under different 
names ; and from the reprehensible practice of changing, 
_ without sufficient reason, the name assigned to a species by 
_ the original discoverer ; an evil, unfortunately, not restricted 
_ to this department of natural history. 
The recent Sponges are arranged in four groups according 
to their structure, viz.— 
Fresh-water Sponges. 
SPONGILLA : siliceous spicula in a translucent jelly-like mass. 
Marine Sponges. 
TerHea : having a tough outer skin ; siliceous spicula in 
bundles, and radiating from the compact nucleus to the 
periphery. 
HauicHonpria : (from silew and cartilago) siliceous spicula 
in a cartilaginous mass. 
GRANTIA : calcareous spicula in a gelatinous mass. 
M. D’Orbigny seems to believe that with the exception of 
the horny Cliona, all the fossil sponges had originally calca- 
reous skeletons,—“ qu'ils n’ont jamais été cornés, mais que 
_ leur tissu a toujours été calcaire et pierreuse ;”* which is cer- 
_ tainly not the case, for abundant examples of fossil keratose 
sponges occur. 
* Cours Elémentaire de Paléontologie, tom. ii. p. 208. 
