40 BRITISH TUNICATA. 



with the movements of the heart. In fact, Van Beneden states 

 that he has seen it do so. I have observed nothing to warrant 

 the belief that either of the channels is in immediate commu- 

 nication with the vascular network of the branchial sac. On 

 the contrary, they both seem to me to open into the pallial 

 plexus, which of course is continued into the wall of the in- 

 halant tube. 



The blood-system does not appear to vary much in the 

 Tunicata ; though certainly I have not traced it in the other 

 genera so completely as in Ascidia, yet enough has been seen 

 to warrant the above assertion. The heart is very similar 

 throughout all the various forms examined ; but its position is 

 not by any means constant. In Corrlla paraUelogramma it is 

 placed on the anterior margin of the stomach, and in con- 

 nexion with the left side of the mantle or inner tunic, follow- 

 ing the removal, in this instance, of the visceral mass from 

 the right to the left side. In Ciona intestinal! s, in which 

 there is developed an abdominal chamber, it is doubled upon 

 itself, and lies in this chamber towards the dorsal margin and 

 between the stomach and the bottom of the branchial sac. 

 The heart in Styela is very long 1 , and narrower than usual ; in 

 this form it lies between the inner tunic and lining membrane 

 on the left, and a little way from the posterior extremity of 

 the mantle, following the curvature of, but at some little dis- 

 tance from, the alimentary tube. The posterior extremity 

 opens into the dorsal branchial channel a considerable way up 

 the endostyle ; the ventral extremity is attached to the 

 stomach, to either side of which it gives a branch in the usual 

 manner. In Pelonaia the heart is likewise in connexion with 

 the left side of the mantle, and in other respects resembles 

 the arrangement in Styela. And in Molgula it holds much 

 the same situation but is placed between the reproductive 

 mass which is above it, and a hollow cylindrical body with 

 hard walls, the nature of which is not understood. 



The branchial sac is usually more complicated than is gener- 

 ally supposed. Hitherto its mechanism has been spoken of in 

 this communication only so far as was necessary to the full 

 comprehension of the blood-system ; it is now time to say 

 something respecting its more minute structure. In all the 

 Tunicates there must- of necessity be present the two great 

 branchial or thoracic channels (the dorsal in connexion with 

 the endostyle, and the ventral at the opposite side of the 

 thorax), even when the branchial sac is only partially or not 

 at all developed ; and in every instance where a true gill is 

 present the transverse channels or primary vessels must also 



