240 INVERTEBRATE PHYSIOLOGY 



than the outer integument, and more attention needs to be paid to the possi- 

 bility of exchanges in this way, particularly since Fox (1952) records 

 anal and oral intake of water in many transparent crustaceans. By keep- 

 ing Carcinus maenas in sea water colored with phenol red, I have found 

 little absorbed in the first few days in intermolt specimens, but much is ab- 

 sorbed during the molt, the phenol red concentrating several times in the 

 foregut and fluid in the digestive glands. 



Some aquatic animals such as coelenterates manage without excretory 

 organs. In fresh water an animal like Hydra, with its large surface in con- 

 tact with the external medium, must regulate its water and ionic content, 

 but the mechanism responsible is practically unknown. Lilly (1955) finds 

 that the ectoderm and endoderm cells of isolated tentacles or whole animals 

 shrink in sucrose solutions stronger than 0.04 M, and believes the internal 

 osmotic pressure is equivalent to that of 0.04-0.05 M sucrose. Using radio- 

 active isotopes of sodium (-^Na) and potassium ("*"K), she finds that 

 these are concentrated in Pehnatohydra, a steady state being reached after 

 12 hours. Accumulation of potassium was more pronounced than accumu- 

 lation of sodium, but the exact localization of the ions in cells or mesogloea 

 was not determined. 



Ionic and Osmotic Aspects of the Composition of Tissues 



An osmotic steady state probably exists between cells and the internal 

 medium, but few direct measurements have been made. In isolated muscle 

 fibers of the bivalve Mytilus, Potts (1952) found the freezing point to be 

 within 1.5% of that of the blood, confirming Krogh's (1939) vapor pres- 

 sure determinations on muscle press juice in the same species. Potts also 

 found osmotic equilibrium between the eggs of the sea urchin Psmn- 

 mechinus and sea water. Organic substances form a large part of the 

 osmotic concentration of cells, and some recent analyses illustrate their 

 importance. 



In leg nerves of Carcinus the amino acids aspartic, glutamic, taurine, and 

 alanine total 271 mM/kg. fresh weight or 313 mM/kg. water (Lewis, 

 1952), forming 28-29% of the total osmotic concentration, assuming it to 

 be that of the crab Ringer solution in which the nerves were dissected out 

 (1096 mg. ions or mM/kg. water). Inorganic ions, acid-soluble phos- 

 phorus compounds, and small amounts of bicarbonate, lactate, and keto 

 acids accounted for another 65% of the concentration, leaving only a 6% 

 deficit unexplained. Lewis has suggested that aspartic and glutamic acids 

 are important in the cation-anion balance of crab nerve, forming 39%. of 

 the total anions. 



The amino acids of crustacean muscle may be present in amounts even 

 greater than those in nerve. Camien et al. ( 1951 ) found about 272 mM/kg. 



