AND PHYSIOLOGY OF THE TUKICATA. 325 



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 trans- 

 verse branchial channels ; while simultaneously the blood will be 

 pushed into the left pallial 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 ramifying in that tunic 

 — the trunk, in fact, that 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 complete when the stream sets in this direction 

 than in the other ; for now the only unaerated 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 oscil- 

 lations in the same direction seem never exactly to agree ; neither 

 is there any constancy as to whether the dorsal or the ventral oscil- 

 lation has the greater number. In a young individual of Ascidia 

 sordida, in which the movements of the heart were carefully ob- 

 served, 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 direction, 70 in the dorsal ; on the 

 second, 64 ventral, 68 dorsal ; on the third, 74 ventral, 88 dorsal ; 

 and on the fourth, 63 ventral, and 64 dorsal. It required 2| 

 minutes to accomplish the beats during a single oscillation. In 

 another individual of the same species, considerably larger than 

 the former, but still quite immature, there were 138 pulsations 

 in one direction, and 120 in the other. Two or three of the con- 

 cluding beats of each oscillation were not so vigorous as the rest ; 

 and when the action was about to change, a dead pause ensued of 

 about two seconds. 



In jPolyclinum aurantium the pulsations were found to be 112 

 in one direction and 115 in the other ; and on starting, the beats 

 were slow. They afterwards became rather rapid, and before 

 ceasing were again retarded ; the action then stopped for a second 



LINN. PROC. ZOOLOGY, VOL. IX. 26 



