114 COMPARATIVE PHYSIOLOGY 



of magnitude as the maximal force which the heart can exert 

 (Straub). 



The same is true of Lamellibranchs. Willen and Minne 

 found that the ventricular pressure rises from lo mm. (water) 

 in diastole to 35 mm. in systole in Anodon. The heart-beat 

 of Anodon is comparatively slow, as may be seen from the 

 following table, for which temperature data are not in all 

 cases available : — 



Laynellibranchsj per min. 



Mytilus . . . . . . 10-15 • • • • Carlson. 



Mya . . . . . . 5-10 . . . . ,, 



Cardium . . . . . . 15-17 . . ■ . ,, 



Anodon 2-4 (at 15° C.) . . Koch. 



Gasteropods : 



Helix 53 Lang 



Cephalopods : 



Octopus . . . . . . 55 . . . . . . Bauer. 



Sepia . . . . • • 35 • • • • • • Fry. 



Koch (19 1 6) has made a very careful study of the heart- 

 beat in Anodon, which has also been recently investigated 

 by Ten Gate (1923). The former investigator describes a 

 phenomenon which is of great interest from the bionomic 

 standpoint, namely the variation of the pulse-rate with open- 

 ing and closing of the shell. In the open condition the heart 

 of an animal may beat five times as quickly as when its shell 

 is closed. Saturation of the water with oxygen also increases 

 the frequency of the heart-beat in Anodon. 



Periodic Reversal of Rhythm in the Tunicate Heart. — ^An 

 interesting field for investigation is met with in Tunicates, 

 whose circulatory system displays a peculiarity which has been 

 familiar to zoologists for nearly a century. The heart in the 

 larger ascidians is an elongated cylindrical tube looped round 

 the base of the pharyngeal sac, so that one end lies dorsal to 

 the gut and may extend to the nerve gangUon, the other end 

 being ventral on the opposite side of the pharnyx. The heart- 

 rhythm is made up of alternating series of beats progressing 

 respectively from the dorsal (advisceral) and ventral (abvis- 

 ceral) ends of the heart. In Ascidia mentula, as described by 

 Day (1921), each pulsation-series lasts from two to four 



