84 Comparative Animal Physiology 



ions in muscle, in skin, and in blood cells are all very different (Table 8).* 



Coelenterates show limited ionic regulation. Jellyfish, for example (Table 

 11), have a cardiovascular cavity containing sea water, but the jelly differs from 

 the sea in the proportion of its ions, particularly in higher potassium and lower 

 sulfate. The total osmotic concentration of the jelly is the same as that of 

 sea water. Sea anemones contain less calcium than does sea water. ^'* 



Ionic regulation in marine molluscs, sipunculids, and annelids has not been 

 much studied. The ratio of various ions inside to outside is near 1.0 for Aplysia 

 and Doris, but the blood calcium is high in Venus and the blood potassium low 

 in Mytilus (Table 11). The sipunculid Phascolosoma maintains its body fluid 

 ions at levels differing greatly from those of ions in sea water. ^" This 

 is the more significant because Phascolosoma has such low salt permeability as 

 to appear semipermeable (Chapter 2, p. 11).^ Some marine polychaetes 

 which adjust osmotically to the medium have nephridia which probably are 

 not used for water regulation, but which may well function in ionic regulation. 



Ionic regulation in arthropods presents a varied picture. Table 11 shows 

 that in sea water the sodium and chloride content of Limuhis, crabs such as 

 Cancer, Callinectes, Maja, and Carcinus, and the lobster Homariis varies by 

 not more than 10 per cent from that of sea water, whereas that of potassium is 

 usually higher. Most of these arthropods also have a calcium concentration a 

 little higher than that of the ocean. The lobster is remarkable in having con- 

 siderably more calcium than potassium in its blood. All Crustacea show mag- 

 nesium and sulfate concentrations much lower than those of sea water. 

 Webb ^-^ presents a series in order of decreasing degree of ionic regulation: 

 Homarus, Carcinus, Cancer, whereas in osmotic regulation the order is Car- 

 cinus, Cancer, Homarus. Arthropods in brackish water maintain the same 

 general proportions among the ions as when in sea water. Callinectes, Mesi- 

 dotea, and Homarus from brackish water have strikingly high calcium content 

 of the blood (Table 11). Fresh-water Crustacea contain high sodium and 

 chloride, which are responsible for their high osmotic pressure. Potassium 

 content is high, and the ratio of magnesium to calcium is higher in the cray- 

 fish than in most marine crustaceans. 



Among vertebrates magnesium content is relatively lower than it is among 

 invertebrates. In summary, ionic regulation at the cellular level does not 

 require ionic regulation of body fluids in aquatic animals; some animals that 

 show osmotic adjustment show ionic regulation; all that regulate osmotically 

 also regulate ionically. 



RESPONSES TO ALTERATIONS IN THE MEDIUM 



Macallum ^^- ^^ pointed out that the body fluids of some marine inverte- 

 brates are like the ocean of today in ratios of the different salts, except for 

 slightly lower magnesium and sulfate; these invertebrates lack circulatory 

 systems (Aurelia) or have an open type of circulatory system (Limulus). Verte- 

 brates differ most strikingly from invertebrates in lower chloride content and 

 hence lower salinity, virtual absence of sulfate, and very much lower mag- 

 nesium content. Macallum suggested that the inorganic ions in the bloods of 

 "higher" animals resemble the ionic proportions present in the ocean at the 



* Since this chapter was written Robertson^"''* has published excellent analyses of 

 dialyzed body fluids of many marine invertebrates. These data show that the potassium 

 concentration of starfish body fluid is significandy higher than that of sea water. 



