BARNACLE, TUNICATE, AND TURTLE. 55 



been unsuspected, and we see that Ringer's solutions, which consist of 

 combinations of sodium, potassium, and calcium chlorides, are only 

 stimulants, and must be partially inhibited and restrained by mag- 

 nesium in order that they may sustain pulsation indefinitely. 



In simple marine animals such as Medusae, barnacles, and Salpa the 

 optimum solution for pulsation is the sea-water itself, but in the higher 

 terrestrial forms the proportions and amounts of the ingredients of the 

 optimum solution have changed, although still composed of sodium 

 chloride, potassium, calcium, and magnesium. In Cassiopea, Lepas, 

 and Salpa it is the special r61e of calcium to assist the sodium chloride 

 to overcome the anesthetic effect of magnesium, whereas potassium 

 practically lacks this power. 



A further inspection of table 6 shows that there are considerable 

 differences in the effects of various elements upon different animals. 

 For example, pulsation is sustained fairly well in Cassiopea, the heart 

 of Salpa democratica, and the loggerhead turtle embryo by a pure 

 NaCl solution, but this quickly stops the movement of the branchial 

 arms of Lepas. Also, the addition of KC1 to NaCl greatly improves 

 the solution in its ability to sustain the pulsation of Cassiopea, whereas 

 it has but little beneficial effect in the case of the arms of Lepas. Cal- 

 cium, on the other hand, has but little power to sustain pulsation in 

 connection with NaCl in Cassiopea, but in the case of the arms of Lepas 

 it is very efficient. In Cassiopea pulsation ceases almost instantly in 

 such non-ionizable solutions as urea, dextrose, and glycerin, but the 

 heart of Salpa democratica will pulsate for a considerable time in these 

 solutions, and the heart of the embryo loggerhead turtle pulsates as 

 long in dextrose as it does in NaCl. These differences in the effects 

 of the several salts upon pulsation in different animals are so consider- 

 able that we must be cautious of drawing general conclusions from 

 the behavior of any one animal and applying them to related forms. 

 For example, Cassiopea can not pulsate for 6 minutes in a solution 

 resembling sea- water but simply lacking calcium, whereas another 

 Scyphomedusa, Linerges mercurius, will pulsate for 45 minutes in the 

 same solution. Both Linerges and Cassiopea are, however, restored to 

 normal pulsation by the addition of calcium, and the difference in 

 their behavior is one of degree, not of kind. The papers of physiol- 

 ogists abound in general conclusions concerning the action of "the 

 vertebrate heart ' ' when only the heart of the terrapin or the dog has 

 been studied, and undoubtedly these sweeping conclusions are often 

 misleading. For example, when the loggerhead turtle embryo is 1 1 to 

 14 days old its heart ceases to pulsate in less than 22 minutes in the 



