406 BULLETIN OF THE BUREAU OF FISHERIES. 
varieties nor strongly differentiated. There are certain cells called ‘“spongo- 
blasts’’ which secrete the material of which the skeleton is composed. Collar 
cells and other epithelial elements line the ciliated chambers and the several 
canals with which they are in communication. The outer surface and the 
subsuperficial or subdermal surfaces are covered with a single layer of flat cells. 
The main portion of the fleshy part of sponges is made up of what is known 
as ground substance, a jelly-like material, similar to that found in the umbrella 
of jelly fishes, without cellular structure, but containing connective tissue cells. 
Muscle cells are found in the skin, the canal walls, and the membranes around the 
peripheral pores, and nervous and sensory cells occur in association with them, an 
explanation of the limited sensitiveness and contractility which are noticed in 
handling live sponges. 
Concerning the life histories of commercial sponges we know but little. In 
some species, at least, the sexes are separate, the females greatly preponderating, 
and the young are produced mainly if not solely from eggs. The young are, for 
a time, minute free-swimming organisms which may be carried considerable 
distances by the currents, and they are still very minute when they at last settle 
down for permanent attachment. At this stage, like oyster fry, they are liable 
to be covered and suffocated by comparatively thin deposits of sediment, and the 
object to which they can successfully attach must be hard and clean. It follows 
from this and from the fact that much of the sea bottom is more or less covered 
with soft deposits, however thin, that a vast majority of the young sponges fall on 
unsuitable bottom and are lost. This accounts in many cases for their irregular 
and sparse distribution on many rocky bottoms which superficial examination 
would indicate as favorable. The natural bars are undoubtedly capable of 
supporting a much heavier growth than they usually bear, and if partially 
grown sponges could be placed on them, as is proposed in the system of sponge 
culture elsewhere described, their productiveness could be enormously increased, 
as these deposits of sediment, fatal to the spat, would prove innocuous to larger 
individuals. 
The rate of growth of sponges under undisturbed natural conditions is not 
definitely known, but the experiments recounted in another connection” indi- 
cate that it is slower than is generally supposed by the spongers. There is very 
good reason to believe that the average annual increase in diameter of sheeps- 
wool sponges in Florida waters is not greatly in excess of 1 to 114 inches. The 
rate varies somewhat in different localities and under different conditions. 
The average 6-inch sponge is probably not far short of four years old, though 
possibly the early growth may be more rapid than the experiments indicated 
for later stages. 
@Moore, H. F., op. cit. 
