ANATOMY AND PHYSIOLOGY. 33 



said to bifurcate, each branch being immediately enlarged, so 

 as to form an elongated caecal vesicle. 



THE BLOOD-SYSTEM. 



The blood-system in the Tunicata is perhaps the most diffi- 

 cult branch in the anatomy to invest'gate ; for these animals 

 are generally too minute and delicate to be successfully in- 

 jected, and it is not easy to obtain living specimens sufficiently 

 transparent to permit of the blood-current being traced 

 through the tissues. Nevertheless much good service has 

 been done in this way by M. Milne-Edwards and others ; but 

 perhaps no one has done more by this method than Dr. Lister, 

 who had the good fortune to meet with a species in every 

 respect suited to the purpose. So far as I have been able to 

 ascertain, the blood-system has been as fully, if not more 

 fully, determined in Perophora than in any other Tunicate. 

 It is therefore satisfactory to find that my results perfectly 

 agree with those obtained by Dr. Lister,* so far as they go. 

 This is particularly gratifying, as the mode of investigation 

 adopted by me is very different from that followed by this 

 distinguished anatomist ; and, moreover, Perophora is one of 

 the Social, while the species used by me are all simple Asci- 

 dians. 



I have relied almost entirely on dissection, aided by the ac- 

 cumulation of blood-corpuscles in the various parts of the 

 system. In this way the minutest ramifications can be traced 

 with the greatest precision. A vast number of specimens, 

 however, are required ; for many individuals may be cut up 

 before one is met with in a proper state. Large specimens, 

 too, are necessary ; and they must have the tissues sufficiently 

 transparent, and the blood-globules opaque or coloured ; in 

 such only can the blood-channels be distinctly traced. And 

 when the specimens are even in the 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 satisfactory ; 

 for in such natural injections there is very little danger of 

 being deceived by the blood being extravasated from its natural 

 channels. 



Art'tli iiinitida and A. cenoxa (fig. 4) are good species for 

 this purpose ; but the one that appears the best-adapted to 

 this mode of investigation is an undescribed species \_A. plan a 

 Hanc.] closely allied to the former. In this the blood-globules 



* Philosophical Transactions/ 1834, p. 375. 



3 



