30 G. H. Parker 



far as I am aware, has ever attempted to measure the pressures 

 involved, it seems useless to urge such objections till actual meas- 

 urement has been accomplished. It is comparatively easy to 

 determine the pressure produced by the activity of the choan- 

 ocytes of such a sponge as Stylotella. To make this measure- 

 ment the following simple device was employed. A glass tube 

 of about five millimeters bore was drawn out at one end to a 

 diameter of about one millimeter, and fixed vertically at such a 

 height that its pointed end was well under the surface of the sea- 

 water in an aquarium. The water of course rose in this tube to 

 the level of that in the aquarium. A long finger of Stylotella in 

 which the currents were running well was ligated at its cut end 

 so that no water could escape from this end, and the osculum was 

 fitted over the small end of the glass tube and firmly tied there. 

 The sponge continued to pump water in through the ostia, and 

 this water naturally rose in the glass tube until the pressure of the 

 column of water in the tube just neutralized the strength of cur- 

 rent produced by the choanocytes. This position having been 

 read on a scale attached to the glass tube, the finger of the sponge 

 was then carefully cut ofi^" from the tube, whereupon the water 

 fell in th6 tube almost to the level of that in the aquarium, the 

 difi^^erence being due to the capillarity of the tube. This new 

 level was then read and the diff^"erence between the twolevels was 

 taken to represent the pressure exerted by the current produced 

 by the sponge. Ten such trials were made and in all cases the 

 readings fell between 3.5 mm. and 4 mm. The current produced 

 by Stylotella, then, has a maximum pressure Equivalent to a 

 column of water between 3.5 and 4 mm. high-. This slight pres- 

 sure is what must be resisted by the tissues, and particularly the 

 sphincters of Stylotella when in a closed conditionof the sponge 

 the choanocytes continue to beat. To ascertain what the resist- 

 ance of the sphincters was, I subjected them to a simple test. A 

 finger of Stylotella in which the ostia were closed was tied as be- 

 fore to the small end of a glass tube which was bent in the form of 

 a siphon and w^as so placed that the end carrying the sponge was 

 in one vessel of water and the other end, quite free, was in another 

 vessel of water. The water in these two vessels was kept at the 



