68 THE BIOLOGY OF MARINE ANIMALS 



Various hydroids and anemones which inhabit brackish water appear to 

 possess low salt concentrations, and to be in osmotic equilibrium with 

 their environment. Internal concentrations of certain ions also follow 

 closely those of the external medium. Sea anemones have calcium con- 

 centrations nearly equivalent to sea water. When placed in dilute sea water, 

 animals absorb water and lose calcium; in sea water plus isotonic CaCl 2 , 

 calcium is absorbed. The body wall of anemones shows two-way perme- 

 ability to water and calcium, but exchange of the latter is rather slow (52). 



The concentrations of ions in coelomic fluids of echinoderms are very 

 similar to those in sea water. Potassium appears to be regulated in all species, 

 and magnesium in some instances. The quantities of protein are very small, 

 less than 1 mg/g of water, too low to affect ionic concentrations. Concen- 

 trations of potassium in the water- vascular system are much higher than 

 in the perivisceral fluid (Marthasterias, Echinus). Suggested mechanisms 

 are, active absorption of potassium via the gills or outward diffusion of 

 potassium through the same structures, thus maintaining a concentration 

 gradient of potassium across the vascular system and coelom to the 

 exterior (9, 112, 113). 



The holothurian Caudina chilensis resembles coelenterates in the facility 

 with which ions and water are exchanged with the environment. Swelling 

 and shrinking take place in hypotonic and hypertonic media. When 

 animals are placed in artificial sea water in which the concentrations of 

 individual ions have been altered, the body fluids alter in conformity with 

 the environment and reach equilibrium within 5 days. Relative rates for 

 ionic movement across the body wall are K + > Na+ > Ca ++ > Mg ++ , 

 and CI- > S0 4 = (69, 70). 



The coelomic fluids of polychaetes, sipunculoids and echiuroids like- 

 wise contain very little protein, below 1 mg/ml of water {Aphrodite, 

 Arenicola and Golfingia). There is slight regulation of ions: potassium 

 is accumulated in most species, and sulphate is reduced in Arenicola and 

 some others. The mechanism of ionic regulation in these animals is un- 

 known. Selective absorption of ions by the body wall, and secretion of a 

 urine low in potassium and rich in sulphate may be involved. It has been 

 suggested that nephridia may be concerned in ionic regulation in these 

 forms. Arenicola marina adjusts in dilute sea water and soon reaches 

 osmotic equilibrium. Potassium and calcium are increased relative to the 

 medium, sulphate reduced, while other ions attain equilibrium with the 

 diluted medium (Table 2.14). Heightened values of potassium and calcium 

 may be due to accentuated ionic regulation in dilute media, or to differ- 

 ences in diffusion rates. An analogous situation is presented by Golfingia 

 muscle immersed in solutions of artificial sea water in which potassium 

 or calcium is altered. Intracellular potassium is kept at a higher level, 

 and calcium is held below that of the external medium over wide ranges 

 of concentrations (133). 



Decapod Crustacea. Marine decapod crustaceans generally possess high 

 levels of sodium, potassium and calcium ions, and reduced levels of 



