MUSEUM OF COMPARATIVE ZOOLOGY. 



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extend transversely from one side to the other. The muscular saddle 

 is larger than the upper, and is attached to it around its entire edge, 

 so that there is a closed empty chamber between the two ; and the 

 lower edges of all three are united, thus forming a crescent-shaped 

 passage between the lower and muscular saddles. When the muscles 

 of the middle saddle are relaxed, as in death by etherization, this 

 layer is drawn up against the upper one, apparently by atmospheric 

 pressure, and the upper closed chamber is no longer visible, while the 



Heart and proximal portion of stolon of young solitary Salpa : b, outer tunic, continued into 

 the outer wall of the stolou ; 1, sinus chambers of stolon; 2, prolongation of pericardium; 

 4, forming the partition of the stolon ; 3, inner saddle of pericardium ; 4, outer saddle of 

 pericardium ; 5, muscular saddle ; G, arrow showing the course of the current through the 

 cavity of the heart. 



lower crescent-shaped channel is open throughout its entire length ; 

 as the heart lies in the sinus system, this channel is of course filled 

 with blood. Now it is plain that if one of the transverse muscular 

 bands which compose the middle layer be contracted, the latter must 

 be drawn down at this point until it comes into contact with the 

 upper surface of the lower saddle, as seen in the figure, in which 

 two such points of contraction are shown. This will of course divide 



