EPITHELIOID MEMBRANES IN MONAXONID SPONGES 559 
Summary. In this sponge the pores do not always close in the 
same way. (1) Often the whole pore canal closes up and dis- 
appears by simple contraction. Pore 2 closes in this way. (2) In 
other cases the pore canal shrinks as before, but actual closure 
is brought about by a rapid extension of the epidermal layer 
across the pore, forming a pore membrane (pore 1). (3) In still 
other cases the pore canal shrinks and closure is then effected 
through the formation of a pore membrane which is gradually 
reinforced by the dermal mesenchyme (pore 3). 
COMPARISON OF THE METHODS OF PORE CLOSURE AS OBSERVED 
IN STYLOTELLA, RENIERA AND LISSODENDORYX 
The various ways in which pores were observed to close in the 
three species of sponges that were studied may be arranged in a 
series expressing the successive physiological states that con- 
ceivably may occur in the closure of a dermal pore in monaxonid 
sponges generally. (1) A partial closure of the pore ma}^ be 
brought about b}^ the extension of the epidermis across the aper- 
ture in the shape of one or more amoeboid strands of protoplasm 
(figs. 12 earlier stages, and 16, for Reniera). Possibly this state 
is sometimes preceded by the formation of fine pseudopodia at 
the margin of the pore (fig. 17, for Reniera). Such closure is 
temporary. The pore opens and then remains open or (2) is 
completely closed by a continuous extension of the epidermis 
across it, fonning a pore membrane (fig. 12 later stages, for Reni- 
era ; figs. 2 and 3 for Stylotella) . (3) To bring about a more secure 
closure of the pore, the pore membrane is reinforced by a centri- 
petal extension of the dermal mesenchyme induced through con- 
traction of the epithelial lining of the pore canal. This reinforce- 
ment extends gradually from the margin across the whole pore 
(fig. 19 and pore 3, for Lissodendoryx) . (4) Hitherto the lower 
part of the pore canal, opening into the subdermal chamber, has 
remained open. Contraction accompanied by centripetal stream- 
ing of the dermal mesenchyme now affects this part of the canal, 
and almost obliterates it (fig. 13, for Reniera) or completely 
obliterates it (pores 1 and 4, for Reniera). 
