AND PHTSIOLOGT OF THE TTJNTCATA. 321 



best condition, many may be opened before the blood-globules are 

 found lodged in the part of the system requiring elucidation. This 

 •method is consequently very laborious ; but the results are satis- 

 factory ; for in such natural injections there is very little danger 

 of being deceived by the blood having extravasated from its 

 natural channels. 



Ascidia mentida and A. venosa are good species for this pur- 

 pose ; but the one that appears the best-adapted to this mode of 

 investigation is an undescribed species closely allied to the former. 

 In this the blood-globules are of a brownish colour and very nu- 

 merous ; so that it sometimes happens that in this animal large 

 portions of the blood-system can be traced in a single individual. 

 Most of the information, on this portion of the anatomy, has been 

 obtained from these three species ; but nevertheless several im- 

 portant points have been verified in the living animal. 



The blood- system in the simple Tunicates may be looked upon 

 as closed, how limited soever the true vascular portion of it may 

 be. The blood-channels throughout the organism are well de- 

 fined ; but whether or not they are provided with proper walls, 

 and, if so, to what extent, is not easy to determine. The trunk 

 channels leading to and from the heart have certainly all the ap- 

 pearance of being true vessels ; and the branchial network has 

 likewise the character of being truly vascular. The blood-channels 

 in the test have also distinct walls ; but in this case they are ap- 

 parently composed of a prolongation of the mantle or inner tunic. 

 Traces, hovrever, of an inner vessel may be observed, in the main 

 trunks ; but this apparent vessel may be nothing more than a 

 continuation of the lining membrane or " inner tunic " of Huxley. 

 In fact, the so-called vascular ramifications of the test, however 

 minute and divided, ought perhaps to be regarded as prolonga- 

 tions of the pallial cavity, although it is quite possible that they 

 carry true vessels ; and, indeed, from the way they are connected 

 with the heart, this would seem almost probable. 



The heart is tubular, and is of considerable length. In Ascidia 

 it is attached to the lower border of the stomach, one end extend- 

 ing some way up the dorsal region towards the intestinal tube ; 

 this may be called the dorsal extremity ; the other, the ventral 

 end, points in the direction of the oesophagus. It lies between 

 the mantle and the lining membrane, within a distinct chamber or 

 pericardium, along one side of which it is attached from end to 

 end. The chamber seems as if formed by a fold of the lining 

 membrane ; -and the heart is probably coated with it in the 



