66 THE PHYSIOLOGY OF EARTHWORMS 



the re-absorption of substances in these regions. Martin (1957) 

 recently discussed these results in the light of more modern 

 knowledge in other invertebrates. He considers that if blood is 

 filtered across the closed nephridium of Pheretima a large osmotic 

 inflow of water would occur, thus forming a hypotonic urine, and 

 causing the animal to handle up to 50% of the body volume of 

 water in 24 hours. Bahl (1947) found a urine production of very 

 nearly this figure (45 %) in animals maintained in a water-saturated 

 atmosphere, equivalent to an excretion of 0-3-0-4 ml/hr/worm. 

 Chapman (1958) considers that a urine production rate very similar 

 to this is shown by Lunihriciis as well. If this is so then the hypo- 

 tonic production may be due to flooding of the nephridium. Elec- 

 trolytes may be actively re-absorbed through the nephridial wall, 

 passing into the coelom (or blood?), and after the elapse of some 

 further period isotonicity would again be established between 

 coelom and blood, (Fig. 21). 



Alternatively Bahl (1947b) indicates that filtration from the blood 

 capillaries surrounding the nephridium may occur, even in 

 situations where they are open to the coelom. This may be 

 facilitated by the beating of the cilia within the tubules. A slight 

 pressure would be set up in the tubule and this may act in drawing 

 liquid through the walls of the nephridium. Carter (1940) 

 previously considered this to be unlikely as the colloid osmotic 

 pressure of the fluid bathing the cells is too great, but Pantin 

 (1947) has shown that the flame cell activity of a nemertine 

 Geonemertes dendyi increases as fluid passes into the animal and 

 osmotic regulatory mechanisms come into play. This sort of 

 action may occur in annelids as well. Indeed Roots (1955) 

 reports that after bathing with hypotonic fluids the cilia of the 

 nephrostome (Lumbricus terrestris and Allolobophora chlorotica) 

 undergo a period of increased activity in rate of beat and sometimes 

 amplitude as well. This is followed by a return to normal. In cases 

 of hypertonicity the ciliary beat slows. 



The question of re-absorption of materials from the nephridium 

 Martin (1957) considered to be unconfirmed, particularly the 

 removal of protein from the fluid in the lumen. He considered that 

 the differences in blood and urine levels of protein, which are in 

 the ratio 16:1 requires that the coelom should provide more than 

 one-sixteenth of the urine filtered. Another objection is that, in 



