54 PTJI^ATION IN BARNACLE, TUNICATE, AND TURTLE. 



IV. PULSATION OF THE BRANCHIAL ARMS OF LEPAS, THE HEART 

 OF SALPA, AND THE HEART OF THE LOGGERHEAD TURTLE. 



The Medusse are the most primitive of the metazoans which display- 

 rhythmical pulsation, and therefore a study of the laws which control 

 their movement is important, for it is practically certain that pulsation 

 began to attain physiological importance in primitive marine animals, 

 and that the vertebrate heart developed in creatures living in salt 

 water. In the most primitive forms the body pulsates as a whole, but 

 finally pulsation is assumed by or restricted to special organs. It is 

 therefore interesting to consider various sorts of pulsating organs in 

 order to see whether some fundamental conditions may not apply to 

 all of them. 



Accordingly studies were made of the pulsation of the heart of the 

 solitary asexual form of Salpa democratica, the rhythmical move- 

 ment of the branchial arms oi Lepas, and the pulsation of the heart of 

 the embryo loggerhead turtle, Thalassochelys caretta, and these varied 

 sorts of pulsation were compared with that of the jellyfish Cassiopea. 



The results are presented in condensed form in table 6 (p. 60) which 

 shows the number of minutes that pulsation endures in various solu- 

 tions consisting of one or all of the ingredients NaCl, KCl, CaClg, 

 MgSOi, and MgClj. In the experiments upon Cassiopea, Lepas, and 

 Salpa Van t' Hoff's sea-water solution was employed. This con- 

 sists of 100 NaCl + 2.2 KCl + 7.8 MgCl^ + 3.8 MgSO, -1- SCaClj, all of 

 y%-a concentration. In experiments upon the heart of the logger- 

 head turtle the proportions of the above-named salts were changed so 

 as to be o. 7 per cent NaCl + 0.03 per cent KCl + o.r per cent MgCla -f 

 0.025 per cent CaClj. The various animals were placed in solutions 

 containing one or all of these salts in the amounts and proportions 

 Stated above. Where + follows a number it means that pulsation occa- 

 sionally lasts a few more minutes than is here recorded, and on the 

 other hand, — following a number means that the pulsation does not 

 usually last as long as is recorded. 



An inspection of table 6 (p. 60) will show that pulsation in all of 

 theselforms Gellyfish, barnacle, tunicate, and reptile) is most powerfully- 

 stimulated by solutions composed of sodium chloride, potassium, and 

 calcium, and that all are depressed by magnesium. Nevertheless 

 sustained pulsation can only take place in a solution containing 

 sodium, potassium, calcium, and magnesium, the last-named element 

 being necessary to " tone down" and restrain the strong stimulation 

 caused by the first three, thus giving a slower but indefinitely sus- 

 ained pulsation. This important r61e of magnesium has hitherto 



