260 INVERTEBRATE PHYSIOLOGY 



pendent of the external salinity, suggests that the ultrafiltration is unaffected (except 

 perhaps in its rate) by the external conditions or the internal conditions resulting 

 therefrom. 



Annelids 



Bahl (1945, 1946, 1947) has made important contributions to our knowl- 

 edge of the physiology of the excretory system of annelids. It cannot yet 

 be said that adequate control of the blood concentration of test substances 

 has been accomplished in any worm, and Bahl's results are somewhat 

 complicated by the strikingly different chloride and osmotic pressure 

 values between blood and coelomic fluid of the earthworm Pheretima 

 posthuma upon which he made his physiological observations. This worm 

 possesses nephridia of three types ; according to Bahl these make up one of 

 the most elaborate nephridial systems of all earthworms. Some are closed, 

 possessing a flame, and of the closed nephridia some empty externally, 

 others into the pharynx and buccal cavity. Some are open through a neph- 

 rostome so that coelomic fluid can pass into the lumen of the nephridium. 

 Those with a nephrostome happen, in this form, to drain into the intes- 

 tines — they are enteronephric. As a result of the number of nephridia and 

 the location of their pores, it was not practical to collect fluid from the ne- 

 phridiopores. Urine was collected either by keeping the worms in water or 

 by allowing urine normally expressed to drip from worms held on a slant- 

 ing glass dish. The atmosphere was saturated with water vapor to prevent 

 evaporation of the urine formed. Fluid extruded from mouth or anus is 

 properly included in work with this species as probably of nephridial 

 origin. 



Bahl was able to collect sufficient quantities of urine by this technique 

 so that analyses could be made of the composition of the urine for com- 

 parison with those of blood and coelomic fluid. The results may be illus- 

 trated most readily with Table 2, adapted from Bahl (1947). 



Inspection of the table shows a striking difference in the salt content of 

 the blood and coelomic fluid, the latter being higher in Na, Ca, Mg, and 

 CI, while the blood has the higher content of K, PO4 and SO4. Even assum- 

 ing a rather low molecular weight for the blood protein, it is still difficult 

 to reconcile Bahl's list of salt concentrations with the freezing-point de- 

 pressions given. It seems likely that other osmotically active substances 

 were not accounted for among the substances analyzed. 



It is of interest to try to understand the differences in concentration of 

 coelomic fluid and blood, since Ramsay (1949a) did not observe a similar 

 striking difference in the worm Luinbricus terrestris. In both studies the 

 animals had been kept in tap water for several days before and during the 

 study. There is ample evidence that earthworms can survive immersion for 



