540 The American Naturalist. [June, 
body, and is carried out through the oscula by the out-flowing 
current, swimming and whirling about in a lively manner. It 
soon assumes a more spherical form, while a depression ap- 
pearing at one end increases in depth until a cup-shaped cav- 
ity results. The young spore then settles on a rock or some 
other substance, mouth downwards, becoming fast to its future 
abiding place. It elongates and becomes a cylindrical larva; 
the depression at the upper end develops into an opening or 
osculum, and the last stage of growth of the sponge is entered 
upon. It has now simply to divide and increase in size to 
form the sponge as we know it. The process varies, of course, 
with different species, but the stages of egg, free swimming 
larva, attached larva and developed sponge are the same in 
all. 
It is, of course, impossible to say that fossil species of 
sponges passed through the cycle which has been briefly de- 
scribed, although there is every reason to believe it to have 
been so. But of one thing we are certain, that in the sponges 
we have a group of organisms which has persisted under a 
great variety of forms through all the vicissitudes of the 
earth’s career. Thousands of kinds have ceased to exist; 
hundreds have been preserved to us in the rocks of various 
formations. Yet with all the extinction that has occurred, 
there is not a single large group which has not both fossil and 
living representatives. It is, therefore, a most interesting — 
group of organisms, and one which neither time nor change- 
ed conditions has caused to disappear. 
The oldest known series of fossil-bearing rocks in the world 
contains forms which belong to the sponges. Like low types 
that live at present, these early sponges were widely dispersed 
over the earth, and the same species occurs in rocks of Lower 
Cambrian age in Labrador on the eastern and in Nevada on 
the western side of the continent. One of the genera that 
seems to combine the features of the two great groups of corals 
and sponges, and whose position is, in consequence, still a 
matter of discussion, is known as Ethmophyllum. The species 
are simple, elongated, cup-shaped, turbinate or club-shaped ; 
they may be curved or straight; ribbed, lobed or corrugated. 
