AND PHYSIOLOGY OF THE TUNICATA. 329 



visceral and pallial plexuses. We have also traced the blood 

 through the principal channels of the organ from one side of it to 

 the other. All, therefore, that remains to be done is to follow the 

 flow of the stream through the minute portions of the structure. 



The extremities of the heart, we have seen, do not open into 

 the ends of the two great branchial channels, but a considerable 

 way above their lower terminations. It is consequently evident 

 that the blood will move upwards in these channels above the point 

 where it enters, and downwards below it ; and when we consider the 

 action of the current so brought to the transverse channels, it is 

 clear that the flow will be in contrary directions in the small 

 longitudinal or secondary vessels above and below this point. Now 

 it has been already stated that on the reversal of tlie action of the 

 heart there is a pause of a second or two, so that for this period 

 the currents cease to move and the fluid becomes perfectly stag- 

 nant. On resuming its function, the first act of the heart is to 

 dilate ; consequently, the blood is drawn towards it from the re- 

 spiratory organ ; and it follows, as a matter of course, that the 

 fluid in the secondary vessels above the point just alluded to in 

 the great branchial channels must flow downwards, and in those 

 below this point upwards. This will be the case whether the 

 blood is brought to the branchial sac by the dorsal or the ventral 

 channel. Such downward and upward set of the blood-current in 

 the secondary vessels has actually been observed in Peropliora by 

 Dr. Lister, who states that " the horizontal vessels were connected 

 also by the smaller or vertical channels between the spiracles — the 

 set of the current in the latter being upwards for the two lower 

 rows, and downwards for the two upper rows." If the heart in the 

 first instance threw the blood into, instead of drawing it from, the 

 gill, the reversal of this motion would take place ; namely, the flow 

 in the secondary vessels above the point indicated would be up- 

 wards, and downwards below it. 



Such are the characters of the branchial sac as seen to exist in 

 A. venosa. The minute network, however, is not continuous 

 throughout the whole organ, but is interrupted in such a manner 

 as to show that it is composed of two lateral lobes or laminae. It 

 is divided along the dorsal line by two parallel folds of the lining 

 membrane, which are separated by a deep groove ; the tissue at 

 the base of each fold is stiffaned by a flattened rod of a somewhat 

 rigid, opake, yellowish substance, which together form the endo- 

 style, that lies, as it were, in the bottom of the groove, along which 

 the rods appear to be united. The upper extremities of these 



