86 



Comparative Animal Physiology 



time when their ancestors' blood became separated within a closed circulation, 

 whereas the fluids of animals like Aurelia and Limulus have changed with the 

 ocean, changes in calcium and potassium occurring^ faster than changes in 

 sodium, magnesium, and especially sulfate. Changes in the environment and 

 in the organisms were thought to be very slow. Macallum suggested that the 

 circulatory system of ancestors of the lobster was cut off from the sea very early, 

 and that possibly the modern lobster came from fresh-water ancestry. Un- 

 fortunately for the theory, the circulatory system of the lobster is open, like 

 that of Limulus. Macallum's general theory was well supported, with respect 

 to the vertebrates, by geological evidence regarding the composition of the 

 ocean at different ages. 



8 10 



c 



12 14 16 18 20 22 24 26 

 Iq (MG/ML) 



Fig. 23. Concentration of chloride in blood (Cli) in mg./ml. as a function of chloride 

 in the medium (CL), in Mesidotea and Astacus. From Bogucki."' "^ 



Fredericq ''^ also emphasized the rapid movement of water and slow 

 changes in ions in the blood of various marine invertebrates on transfer to 

 dilute media. 



Tests of Macallum's hypothesis and of the limitations imposed by salt 

 balance on animal distribution have been made by placing aquatic animals in 

 sea water of different tonicities, or in solutions containing ions in proportions 

 differing from those in the customary media, then measuring the changes in 

 composition of the animals' blood. 



The effects on ionic composition of transfer to different concentrations of 

 sea water are shown for several crustaceans in Table 12 and Figures 23-25. 

 When fresh-water crustaceans such as Astacus or Telphusa or a fresh-water 

 Eriocheir is transferred to various dilutions of sea water, numerous changes 

 occur. The water content of blood and of muscle may diminish slightly; the 



