Reactions of Sponges 5 



ney-like membranes around the oscula very generally collapse 

 showing that they are delicate structures. That the normal 

 closure of the osculum is not a purely mechanical collapse of this 

 kind is seen from the tact that when a sponge in which the closure 

 has taken place by gradual exposure to air is returned to sea- 

 water, its oscula do not flap open but can be seen to expand only 

 gradually as by the relaxation of a sphincter. It is quite clear 

 that the closureof the osculum is a definite response, which, among 

 other thmgs, prevents the entrance of air mto the cavities ot the 

 sponge when, by a fall of the tide, the sponge becomes exposed 

 to the air. 



Another response which can be observed in Stylotella in its 

 natural state is seen on comparing specimens that have been 

 exposed for some time to the air on the beach with specimens still 

 in the water. The latter as a rule have a plump appearance and 

 a relatively smooth surface, whereas those that have been in the 

 air look somewhat shriveled, and their surfaces are roughened as 

 though their flesh had shrunken down on a rather resistant skele- 

 ton. At first sight it would seem that the sponge had shriveled 

 simply because under action of gravity the water had been 

 drained from it, but that this shriveling is probably not thus 

 produced, but is dependent upon a positive contraction of the 

 flesh of the sponge, is seen from the fact that the same shriveled, 

 rugose appearance can be assumed by a sponge in seaiuater under 

 conditions to be described later. These two reactions, the clos- 

 ing and opening of the oscula, and the shriveling and filling out 

 of the common flesh of the body, are the most obvious Natural 

 responses exhibited by Stylotella. 



3. STRUCTURE OF STYLOTELLA 



Stylotella is an encrustmg sponge that usually throws up longer 

 or shorter fingers (Fig, i.) These fingers, which represent the 

 individuals in the colony, may attain a length of four centimeters 

 and each one carries near its distal end usually one, sometimes 

 two or more oscula. When fully expanded the oscula are roundish 

 openings in a dome-shaped elevation or at the end of a more 



