SPONGES 15 
The method of formation_of the water system is difficult to make out, but 
a smaller tubular system appears to penetrate through the sarcodeat t! c ex- 
treme end of the cylinder, among the growing fibers of the skeleton, the: the 
tovgh incurrent tubes appear to be formed in the skin of the sponge, outside 
the lining membrane and closing membrane of the mouth of the central tube, 
and then penetrates to the nearest newly-forme d small tube, that con iects 
with the older water system. Then the small tubes become gradually ealorged 
into the regular cellular system. 
Fig. 4. 
«.moeba. Ideal Cilia cell of Sponge. 
The smooth lining membrane of the central tube is about .10 in thick- 
ness, and near the terminal end of the tube, it 1s projected forward to form 
the circular closing membrane, which, at the point of jointure with the regu- 
lar lining membrane from which it arises, is considerably thickened, but be- 
comes thinner on the inner edge. See Fig, 6 and 7 above o, both figures 
being life size. 
The cells into which the incurrent tubes open, are provided with cilia 
or vibratory organs which are quite likely thrust out from the flesh forming the 
walls of the cell, much as are the pseudopoda, or false feet, of the amocbua. 
