SPONGES 13 
cle, by which the opening is closed, in fact, the opening appears to remain 
closed, unless opened by pressure of fluid from within. These are excurrent 
openings, and they communicate through a short cylindrical tube, having soft 
walls, often directly in contact with the last cell of the water system. ( See Fig. 
3, D, and also c in Fig. 2, A and B. 
The incurrent orifices are about .25 apart, on the average, and are scat- 
tered quite irregularly over the surface, varying in number from 12 to 16 to 
the square inch, (See Fig.y where! have given the actual number toa square 
inch of the surface of a cylinder, which was about eight inches long, showing 
Fig. 2: Fig. 3. 
F E a 
A B om D 
Water system of Tube Sponge considerably enlarged. —_ Allen’s Harbor, May, 
]893, 
