on the Natural Group of Tunicata. 545 



mach in a spiral form may so be considered. The stomach is 

 internally simple, with very slight transverse striae. 



The heart is situated horizontally between the lower part of 

 the tunic and the stomach. It is large, ovoidal, and appears to 

 be composed of several lobes, and is indeed of a structure diffe- 

 rent from that of such Ascidida as are known. There are four 

 vertical openings to it, which are capable of being widely dis- 

 tended. ..■'1 .v,....-^.\ '. 



The dorsal sulcus is remarkably distinct, and proceeds from 

 the immediate vicinity of the heart, or rather along the back of 

 it, to that of the branchial orifice. It may be seen through the 

 external envelope of the body, when this is viewed on the left 

 side, and forms an arch enclosing a lesser and more pointed 

 arch, which last appears to be nothing else than one of the folds 

 of the branchial pouch. At the point where this last arch touches 

 the dorsal sulcus there is in our specimen an orifice opening ex- 

 ternally, and apparently communicating by a tube with a beau- 

 tifully diaphanous longitudinal pouch, which contains nothing 

 but two blackish nodules, one of which is longer than the other. 

 The imperfection of the only specimen in my power to examine 

 has prevented me from accurately ascertaining the nature and 

 use of this organization, which appears to have nothing similar 

 to it in any of the other Tunicata hitherto observed. 



The third animal whose structure I have to explain belongs to 

 the natural 



Genus ASCIDIA Linn. Cynthia Sav. 



External Character. Boc??/ sessile, with a coriaceous test, 



having both orifices quadrifid, or at least having the 



anal orifice very rarely transversely cleft. 



Ana- 



