SPONGES 11 
membrane of the water system, and may not only be removed entire, but also 
often remains intact after the sarcode has been marcerated away. ( See Fig. 2. 
A, B, where I have given an enlarged cut of ‘this tube ; c, is its opening. ) 
As seen, this orifice is not guarded by any sphincter, or “other membrane or 
valve. In fact, the rigidity of the walls show that the mouth of the tube is 
constantly open. 
The incurrent tubes open into cells which vary in diameter, from .10 to 
.25. Usually the upper cell is a little longer than wide, and opens into another 
similar cell, deeper in the walls of the sponge, and this into another, and so 
on through a series of from four to seven, or even more, the last resting against 
the lining membrane of the central tube. (See Fig. 2 a, a,a, a, being the 
incurrent tubes with the cells below them. ) Sometimes the opening between 
the cells is partly closed by a kind of translucent tympanifo1m membrane. 
that is stretched from one twig of the skeleton to another. ( See Fig. 2, B. 
v.v, v, where I have given a cut of these tympaniform membranes enlarged 
about three diameters, and as will be seen the form varies. ) 
From the sides of many of these cells, open small tubes which commu- 
nicate, often in a winding manner, either with some other similar tube, or with 
another system of cells, but without any regularity. (See Fig. 2, where | 
have given two of these tubes, connecting two cell systems, and the black dots 
in the cells show the entrance to other tubes, and others maybe seenat A, 7. 
tT, and 0, o, in B. 
The surface of the membrane lining the great central tube of the sponge 
is smooth, and scattered very irregularly over it are openings, which are not 
situated upon tubecules, and which are provided with a kind of sphincter mus- 
