70 THE PHYSIOLOGY OF EARTHWORMS 



marine animals, that of dehydration, but for a different reason. In 

 marine animals water is lost to the greater external osmotic 

 pressure; in terrestrial animals water is lost by evaporation from 

 the body surface, and by leakage from the various natural pores of 

 the body. Conservation of water is, therefore, of prime importance, 

 particularly in those animals living in regions where water is 

 scarce. In many animals the excretory products have changed from 

 the highly soluble, but highly toxic ammonia and trimethylamine 

 oxide, in favour of the less soluble but also less toxic substances, 

 urea and uric acid. These require less water for their removal and 

 thus excretory processes do not denude the animal of very 

 necessary water supplies. 



Oligochaetes are found in fresh water and upon land. We thus 

 expect to find appropriate modifications in their physiology to deal 

 with the special strains in water balance imposed by these habitats. 



As mentioned previously (p. 2) the earthworm body contains a 

 very large amount of water. The average water content amounts to 

 about 85% of the fresh weight of earthworms. A considerable 

 proportion of this total is present as coelomic fluid or as blood, and 

 a further large fraction is found within the gut in the form of 

 secretions. Water can escape from the body via the mouth and 

 anus, the dorsal pores and nephridia and as mucus, either singly 

 or in combination. Very little is known at present about the water 

 relations of fresh-water oligochaetes such as TuhifeXy though it is 

 to be expected that water passes in through the body wall and that 

 the animal excretes considerable quantities of hypotonic urine in 

 common with many other fresh-water animals, and similarly to 

 earthworms. 



The terrestrial oligochaetes Liimbricus and Eise?na have only a 

 thin cuticle overlying a mucus secreting membrane, the epidermis. 

 It is unlikely that the cuticle plus epidermal layer provides a 

 serious barrier to the influx and efllux of water through the body 

 wall, so that changes in environmental conditions can lead to 

 considerable changes in the w^ater balance of the animal. 



Earthworms dug from the soil are not fully hydrated and will 

 increase in weight if immersed in tap water. They may gain up to 

 15% in weight in 5 hours, losing it again when replaced in soil 

 (Wolf, 1938, 1940). The loss of weight in soil is a faster process 

 than the gaining of weight in water (Adolph, 1943). These minor 



