38 BRITISH TUNICATA. 



lation of the branchia will be hurried into the great ventral 

 channel, and by this to the ventral extremity of the heart. 

 But before it reaches so far it will be joined by the streams 

 derived from the visceral plexuses of both sides of the body, 

 and in this way with that from the pallial plexus, chiefly, of 

 the right side. The greater portion of the blood from the 

 left side of the mantle will reach the heart by the ventral 

 branchial channel, having been brought hither by the sus- 

 penders. The blood thus returned will likewise have com- 

 mingled with it that which is drained from the vascular 

 system of the test by the ventral trunk of that system. It is 

 thus apparent that the blood which arrives at the heart in 

 this direction is only a partially aerated current. 



When the action of the heart is turned in the opposite 

 direction, just the reverse of all this takes place. The blood- 

 current will now fill, in the first instance, the visceral plexuses 

 of both sides, then the right pallial plexus; at the same time 

 it will reach the great ventral channel of the branchial sac, 

 and through it the transverse branchial channels ; while 

 simultaneously the blood will be pushed into the left pallia] 

 plexus through the suspenders placed along the ventral 

 channel. The blood that now enters the vessels of the 

 branchial sac will be joined by numerous streamlets issuing 

 from the suspenders, and brought by them out of the visceral 

 and pallial plexuses, and will ultimately arrive in the great 

 dorsal channel, and so to the dorsal extremity of the heart, at 

 which point it will be mingled with the current from the test 

 brought by the dorsal branch of the compound vessel ramify- 

 ing in that tunic the trunk, in fact, which in the first 

 instance carried the blood to the test. Here, then, as well as 

 in the former case, the current returned to the heart is only 

 in part aerated ; but the aeration is undoubtedly more com- 

 plete when the stream sets in this direction than in the other ; 

 for now the only uiiaerated portion is that from the test, 

 while in the first case the blood from the visceral and pallial 

 plexuses is likewise in a partially aerated condition. 



The pulsations of the heart appear to vary considerably in 

 number even in the same individual; and the numbers of the 

 oscillations in the same direction seem never exactly to agree ; 

 neither is there any constancy as to whether the dorsal or the 

 ventral oscillation has the greater number. In a young indi- 

 vidual of Ascidia sordida, in which the movements of the 

 heart were carefully observed, the pulsations were counted 

 four times in each direction, and the following was the result. 

 On the first occasion there were 73 beats in the ventral direc- 



