486 B. A. MINOHIN. 



to regard the pore-cells as constituting a distinct class of cell- 

 elements. 



If the pore-cells always remained expanded, it is inconceiv- 

 able that any misconceptions should have arisen as to their 

 true nature. But unfortunately, perhaps, for the student of 

 spouge literature, the pore-cells, like the epithelium from which 

 they originally sprang, are very contractile. On the least 

 provocation the pores close up, and then present an appear- 

 ance which has led to much confusion. The external aper- 

 ture seems to be the part first affected, the opening disappear- 

 ing by contraction of the delicate membrane in which it is 

 situated, like the closing of an iris diaphragm. Hence pore- 

 cells occur commonly which show a widely open gastral 

 aperture, but no trace of the dermal opening (PI. 42, fig. 50). 

 Next, the gastral opening narrows itself, and finally closes 

 up, and the result is a large compact granular cell, of a 

 type very familiar to all students of Ascon histology, since in 

 all specimens not completely expanded these cells are the 

 largest and most conspicuous elements in the sponge. It is, in 

 fact, far easier to obtain preparations showing the pores closed 

 than to obtain them with the pores open. In order to show 

 the open pores, as in the preparations represented in PI. 38, 

 fig. 10, and PI. 41, fig. 39, it is necessary to preserve fully 

 expanded specimens of the sponge as soon as they are found. 

 If, on the other hand, the sponge be brought back to the 

 laboratory and there preserved, every pore will be found, as a 

 rule, completely closed, and no longer recognisable as a pore 

 except by a comparison with preparations showing pore-cells 

 in the expanded state. PI. 41, fig. 38, though showing pore- 

 cells which have not as yet acquired an opening, may be taken 

 as representing the characteristic appearance of the closed 

 pores, and fig. 39 also shows one such cell. It is hardly 

 necessary to state that in all their cytological characters these 

 cells agree with what has been said above for the pore-cells, 

 and differ from them only in form ; their compactness makes 

 their yellowish-brown tint even more obvious than is the case 

 when expanded. 



