REPORT ON THE TUNICATA. 279 



separated off from the Cynthinse, since the latter group became distinct from the Styclinaa 

 through the development of compound tentacles. Next comes the modification of the 

 branchial sac, a change consisting in the suppression of the interstigmatic vessels, which 

 appears to have taken place in the Styelinae and the Bolteninse independently, and 

 resulting in the evolution of two forms (E. and E'. in the scheme), which were tin- 

 ancestors of Bathyoncus on the one hand and of Culeolus and Finujuhis on the other. 



The above is what seems to me the simplest and most natural method of accounting 

 for the structural relations of the present genera, the only difficulty being the independent 

 origin of the modified branchial sac in two distinct groups. And the difficulty is increased, 

 since it is not evident what is the advantage of the structure seen in Culeolus, or what is 

 the reason of the modification. The idea that that structure is possibly better suited to 

 certain conditions consequent upon living at great depths is probably not correct, since 

 we find other Simple Ascidians, from as great or greater depths, having the normal struc- 

 ture of branchial sac, such as Abyssascidia ivyvillii, Corynascidia sidirni, Sty el a squamosa, 

 and Styela bythia, although it is true these forms are, according to my phvlogenetic 

 scheme (page 286), all less highly developed than at least Culeolus and Fungvlus, and 

 probably than Bathyoncus also. 



A very notable feature in Culeolus is the condition of the blood-vessels in the test in 

 some of the species {e.g., Culeolus murrayi). This structure, when I first saw it in section 

 (PL VIII. fig. 2), at once suggested to my mind the idea that it was an accessory res- 

 piratory apparatus, and I still incline to that view. It is not difficult to imagine how the 

 blood-vessels in the test might, especially if the supply was not abundant, become branched 

 in the superficial layers of the organs, and swollen in their end twigs, in order that the blood 

 circulating in the test might thus receive a little additional aeration, and in this way the 

 large blood- vesicles and hollow papillae of Culeolus murrayi could be explained. Possibly 

 the enlarged terminations of the vessels seen in the tests of so many other Ascidians may 

 not only be the same structure in a less developed condition, but may also perform the 

 same function in a slighter degree. A glance at the diagram of the circulation in a 

 Simple Ascidian (page 280), will show that when the heart contracts ventro-dorsally 

 the test receives almost pure blood, but that when, on the other hand, it contracts dorso- 

 ventrally the blood carried to the test is impure blood, which has been returned from the 

 viscera, and is on its way to the branchial sac. From a consideration of this arrangement, 

 it is obvious how advantageous it would be for the Ascidian if the test could act, even 

 to a slight degree, as an accessory respiratory organ, and could allow the blood circulating 

 in its superficial layers to be brought into such close relation with the external medium 

 as to render possible a certain amount of oxydation. 



In those forms where the terminal twigs of the blood-vessels are prolonged into 

 delicate processes of the test, these hair-like structures have generally acquired a second 

 function — that of attaching to their surfaces sand grains, small stones, shell fragments, and 



