42 BRITISH TUNICATA. 



is not very easy to determine of what use they are; but per- 

 haps their chief function is to protect the more delicate tissue 

 of the true aerating vascular surface ; Avhile the papillae will 

 conduce to the same end, and by the aid of their cilia prob- 

 ably sweep the sedimentary matters towards the oral lamina, 

 the water being- beat through the stigmata by the cilia that 

 fringe their borders. From the stiffness of the bars them- 

 selves it may be inferred that they will also give support to, 

 and keep stretched out, the vascular network of the sac. 

 They seem ill calculated, on account of the thickness of their 

 walls, to give much assistance in aerating the blood, and are 

 certainly unnecessary as part of the circulatory mechanism. 



The blood, as we have already seen, is brought to and taken 

 from the aerating reticulation by the dorsal and ventral bran- 

 chial channels, and by numerous suspenders connecting it 

 with the 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 con- 

 siderable way above their lower terminations. It is conse- 

 quently 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 the 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 stagnant. On resuming its function, the first act 

 of the heart is to dilate ; consequently, the blood is drawn 

 towards it from the respiratory 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 Perophora by Dr. Lister, who 

 states that " the horizontal vessels were connected abo by 

 the smaller or vertical channels between the spiracles the 

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



