35 Comparative Animal Physiology 



hr. in an animal weighing 50 grams (without shell). Fluid collected from the 

 kidney (organ of Bojanus) is lower in chloride and calcium but higher in 

 protein and nonprotein nitrogen than the blood. '^^ Hence the kidney reab- 

 sorbs some salt but still much is lost in the urine. The salt intake by food 

 could supply only a small fraction of this amount and, furthermore, fresh- 

 water mussels will live well for many months in running tap water without 

 food, with little change in blood concentration. "'* To compensate for the 

 salt loss it is likely that salt is actively absorbed. Anodonta and Unio can 

 actively absorb chloride and sodium from approximately millimolar solutions, 

 but they cannot reduce the concentration below about 0.1 mM.^^"'^ Krogh 

 suggests that when salt absorption by che body surface is prevented, as in 

 distilled water, the nephridial salt reabsorption may be increased so that the 

 urine concentration may approach zero. 



The kidneys of most gastropods and pelecypods are connected to the peri- 

 cardial cavity and may receive pericardial fluid directly into the kidney lumen. 

 In addition, the kidneys receive a rich supply of blood returning from visceral 

 sinuses, and the epithelium in some regions is highly vacuolated and secretory. 

 The hydrostatic pressure in the postrenal sinus may even exceed pressure in 

 the pericardium (see Chapter 15). Urine volume cannot, therefore, be calcu- 

 lated from pericardial filtrate or urine composition be inferred from the fluid 

 from the interior of the kidney. A histophysiological study of the molluscan 

 kidney would be useful. 



Snails show wide fluctuations in osmotic concentration according to activity 

 and water supply. ""'^ The snail Limnaea has a blood concentration equiva- 

 lent to 0.43 per cent NaCl and urine concentration of 0.3 per cent NaCl. -°^ 

 Limnaea is able to absorb salt actively from 0.01 Ringer solution. '^^^ The 

 land snail. Helix, has blood equivalent to 0.69 per cent NaCl when hiberna- 

 ting and to 0.5 per cent NaCl when active. ^-'^ The blood concentration of 

 a land snail dropped from a normal value of Ai=0.47 to Ai=0.2 after a 

 rain. ^- 



Earthworms and Leeches. There are many fresh-water annelids, specifically 

 Hirudinea and OHgochaeta; the earthworms are adapted to life in moist soil 

 where osmotic stress is intermediate between that of fresh water and that of 

 air. Soil-dwelling earthworms can live indefinitely in aerated fresh water or 

 in moist air. Adaptive mechanisms of water balance in earthworms have been 

 examined in detail.'^- ^' ^^- ^'^^' -"=«• '^^ 



The blood of freshly collected earthworms is similar in osmotic concentra- 

 tion to the blood of aquatic vertebrates; Af.p. of Ltivihrictis body juice is 0.31°,-'' 

 of blood plus coelomic fluid 0.59°,^^^ of coelomic fluid of worms in distilled 

 water 0.3°;2"»« the Af.p. of Pheretima blood is 0A°-0.5°.'^^ There is much 

 variation in osmolar concentration according to the state of hydration. 



When an earthworm is transferred to tap water it absorbs water equivalent 

 to as much as 15 per cent of its initial weight in about 5 hours. Water-adapted 

 worms removed from aerated fresh water to moist air or soil lose water.-''-- -^'-^ 

 Hence under natural conditions an earthworm is always in a semi-desiccated 

 state.^' '■'' In dry air earthworms can survive a loss of 70 to 80 per cent of 

 their body water. "^' -^"^ Water goes out of the worm after excess hydration 

 more quickly than it is taken in after dehydration. " Earthworms orient 

 toward a moist and away from a dry surface. -^^ 



