306 G. p. BIDDER 



a pressure there which forces an equal amount of water out 

 into the efferent canal (Text-fig. 7), and so the pressure created 

 in the chamber is transmitted to the efferent canal, and thence, 

 with a loss by friction, to the cloaca. The chamber is distended 

 by this flagellar pressure, as the elastic bag of a squeeze-pump 

 is distended, and the stretching of the chamber-walls is resisted 

 by the surface tension and elasticity of the tissue, as the 

 stretching of a soap bubble is resisted by the surface tension of 

 soapy water in air. The text-books have long pointed out that, 

 as the canal system is specialized, the diameter of the chambers 

 become smaller, and they change from cylinders to hemispheres 

 and spheres. This change, therefore, directty increases the 

 possible pressure in the chambers, and therefore the diameter 

 of supply. There is twice the pressure in a soap bubble 1 in. 

 in diameter that there is in a soap bubble 2 in. in diameter ; 

 and similarly, reduction in the size of flagellate chambers pro- 

 portionally increases the pressure which their tension can 

 balance. In Leucandra I calculate from computation of the 

 oscular current ^ and the friction in the canals that the pressure 

 is between f mm. and H mm. of water in the cylindrical 

 flagellate chambers 54 ju, wide. The same tension would 

 support double the pressure in spherical chambers of the 

 same diameter, and four times the pressure in spherical chambers 

 of half the diameter ; so that from my results the pressure in 

 a sponge wuth spherical chambers 27 fj- in diameter would be 

 2| to 5 mm. of water, and in 35 ix chambers would be 2 to 

 4 mm. In spherical chambers of 35 ix, by direct experiment, 

 Parker found the pressure of 3| to 4 mm. of water in Stylo- 

 tella. We may therefore conclude with some safety that 

 the pressure in Leucandra is close to 1 mm. of water, 

 and that the healthy tension of the chamber-wall tissue 

 is nearly alike in this and in Stylotella (for the latter 

 0-00034 gm. weight per centimetre, or less than gV of the 

 surface tension of petroleum in water). In the smaller spherical 

 chambers of Stylotella this tension can support three times 

 the pressure of the large cylinders of Leucandra, and so 

 ^ See Appendix, Note 6. 



