REPORT ON THE TUNICATA. 01 



In Ascidia the heart lies upon the left side of the body, along the posterior edge of 

 the stomach, while in C 'to na it is also in close relation to the stomach, but runs more 

 antero-posteriorly than in Ascidia, on account of the changed relations of the alimentary 

 canal. In Corella the heart is placed comparatively far forwards, being situated upon 

 the anterior border of the stomach, and consequently upon the right side of the body. 



In Clavelina the heart is placed in the abdomen, and runs antero-posteriorly along 

 the side of the stomach on its inner edge. 



In the Ascidise Composite the heart varies somewhat in position in different groups, 

 and, according to Milne-Edwards, is always in close relation to the generative organs. In 

 Botryllus, where there is no abdomen developed, both heart and genitalia are situated close 

 to the posterior part of the branchial sac, the heart being on the left side. In Didemn urn and 

 some allied forms, the heart runs alongside the stomach on the right side of the abdomen, 

 while in Amaroucium and a number of other genera, the heart and the genital organs form 

 a long projection, extending far beyond the intestine, and known as the post-abdomen. 



In Pyrosoma the heart is placed towards the posterior end of the ventral edge of the 

 branchial sac, and near the stomach. 



In Salpa the heart is large, and is situated close to the visceral nucleus on its anterior 

 and ventral sides. 



In the Appendiculariidge the heart only is known. It is a short sac placed upon the 

 ventral side of the stomach, between the two lobes, and having longitudinal slits in its 

 sides. Neither Huxley nor Fol have been able to observe blood corpuscles. 



In the forms in which it has been most completely made out, the circulation is as 

 follows: — A large vessel arises from each end of the heart (fig. 13, h.). One of these (br.c), 

 after giving off a branch to the test (v.t!.), runs along the ventral edge of the branchial 

 sac below the endostyle (v.v.), and gives off lateral branches which open into the transverse 

 vessels (tr.) of the branchial sac, and thus distribute the blood to be aerated. The heart 

 itself is in all probability merely the modified posterior end of this great ventral vessel. 

 The trunk arising from the dorsal end of the heart (cs^j.) gives off first a branch to the 

 test (v.t.), which turns towards the corresponding branch from the ventral vessel, and the 

 two pass side by side through the mantle, to enter the test usually near the posterior end 

 of the ventral side. In the substance of the test they break up into branches, which run 

 alongside one another, and finally communicate in the terminal knobs (t.k.) of the ulti- 

 mate twigs. The dorsal trunk then gives off branches to the mantle (v.m.), and ends by 

 breaking up into a number of vessels for the stomach (v.st.), intestine, and reproductive 

 organs. 



The blood circulating in the mantle, the stomach, the intestine, and the reproductive 

 organs is then collected by several vessels (sp.&r.) opening into a large trunk (d.v.) lying 

 along the dorsal edge of the branchial sac, and communicating with the dorsal ends of the 

 transverse vessels of the sac. Thus, as Lacaze-Duthiers has pointed out, there are three 



(ZOOL. CHALL. EXP. PART XVII. 1882.) R 8 



