78 



THE PHYSIOLOGY OF EARTHWORMS 



the bathing saUne was replaced by 045% NaCl first the narrow 

 tube and later the middle tube opened and the cilia started to 

 beat again. The bladders became filled after about 30 minutes 

 (Ramsay, 1949b). 



Ramsay's results show that almost certainly no change occurs in 

 the composition of the urine within the narrow tube. The osmotic 

 pressure of fluid here is unchanged from that of the external fluid, 

 indicating perhaps that water and salts pass in both directions 

 across the wall with equal facility, or that filtration is occurring 



250 



200 



250 



100 



50 



r^r-^ 



Fig. 23a. To show the osmotic pressure of urine at different 

 levels in the nephridium. The osmotic pressure of the ringer sur- 

 rounding the nephridium has been equated to 100. 1. Nephridios- 

 tome. 2. Narrow tube. 3. Middle tube. 4. Wide tube proximal. 5. 

 Wide tube middle. 6. Wide tube distal. 7. Bladder. 8. Exterior 

 (from Ramsay, 1949b). 



across the wall of the blood vessels around the organs into the 

 lumen of the tubule, or that coelomic fluid passes directly through 

 the open nephrostome and from there is passed along to the middle 

 tube. In this region more variable results were found, but in 

 general a slight decrease in osmotic pressure was found. This drop 

 was more pronounced, however, in the next section of the nephri- 

 dium, the ampulla of the wide tube. In this region the osmotic 

 pressure may be as much as halved compared with the external 

 medium. At the distal end of the wide tube the osmotic pressure is 

 even less, suggesting that the property of forming a hypotonic 



