WATER RELATIONS 73 



If earthworms are allowed a choice between water-saturated or 

 moist, air-filled soil, the very wet areas are avoided by Dendrobaena 

 subrubicunda and L. terrestris but A. longa, A. caliginosa and A. 

 chlorotica are sometimes found in the waterlogged soils. This last- 

 named species is interesting as it exists in two colour forms, one 

 pink and apparently living in drier soils, whilst the other is 

 green and often found in very wet areas, such as on the muddy floor 

 of Lake Windermere (Kalmus et aL, 1955). In laboratory studies 

 no difference was found between the soil preference of these two 

 forms (Roots, 1956). 



The avoidance reaction of earthworms to water has been 

 suggested as the causal agent in the well-known evacuation of 

 burrows by certain species after heavy rain (Roots, 1956) first 

 noted by Darwin (1881), but recordings of the electrical activity in 

 the segmental nerves of L. terrestris and A. longa indicate that 

 water does not stimulate sense organs in the body wall (Laverack, 

 1960b), although this does not preclude the possibility that water 

 entering the body cavity through the skin dilutes the coelomic 

 fluid bathing the central nerve cord, increasing the activity within 

 the nerves in a manner analogous to that noted in slugs (Kerkut and 

 Taylor, 1956). In this connection it has been noted that anaesthesia 

 or extirpation of the nerve cord leads to a breakdown of the 

 osmoregulatory powers of another annelid, the leech Hiriido 

 medicinalis, and that this appears to be a function of the whole 

 nerve cord and not a particular portion (Ros9a, Wittenberger and 

 Rusdea, 1958), but preliminary results on L. terrestris have been 

 negative (Laverack, 1960 unpublished). 



The ways in which an earthworm maintains a constant, or 

 reasonably constant, internal medium has been the subject of some 

 disagreement until recently. The actual value to be attached to the 

 osmotic pressure of the coelomic fluid has been reported to vary 

 widely, and this may be a reflection of the bodily conditions of 

 animals taken fresh from soil sites of different properties. The 

 freezing-point depression of the coelomic fluid in Lumbricus is 

 approximately 0-31 °C (Adolph, 1927 and Ramsay, 1949a), whilst 

 that of Pheretima posthuma lies between 0-28 and 0-31 °C (Bahl, 

 1947a). The osmotic pressure of the blood oi Lumbricus is slightly 

 below that of the coelomic fluid (Ramsay, 1949a) but in P. posthuma 

 the freezing point depression of blood is 0-4-0-5 °C indicating that 



