. Nature . its chief function, and probably itsonly one, is the asexual production of new individuals. The ancestors of the remaining Simple Ascidiansdiverged from the ancestors of the Clavelinidae, and lostthe power of reproducing by gemmation, but in many ofthe least modified of the Ascidiidae we still find processesfrom the posterior end of the test which contain vessels,and so closely resemble the stolon of Clavelina in allparticulars that there can be no doubt that they are per-sistent rudiments of that structure. In Ciona, which is certainly one of the most primitiveof the Ascidiidae, ve
. Nature . its chief function, and probably itsonly one, is the asexual production of new individuals. The ancestors of the remaining Simple Ascidiansdiverged from the ancestors of the Clavelinidae, and lostthe power of reproducing by gemmation, but in many ofthe least modified of the Ascidiidae we still find processesfrom the posterior end of the test which contain vessels,and so closely resemble the stolon of Clavelina in allparticulars that there can be no doubt that they are per-sistent rudiments of that structure. In Ciona, which is certainly one of the most primitiveof the Ascidiidae, vessels are only present in the posteriorpart of the test, and here we frequently find them drawnout into long processes of the test, which have the greatestpossible resemblance to stolons (Fig. 2), and are doubtlesstheir homologues, although they no longer function as bud-producing organs. They are useful as adhering organs,and they have probably to a slight extent commenced toperform a respiratory Fig. 5.—Vessels in the surface layer of the test of Ascidia mannnulata asseen in a section magnified about 40 diameters, b, small part of the systemmore highly magnified; , afferent vessel; , efferent terminal knob; f., surface of the test. I imagine then the first stages in the evolution of the respiratory vessels to be as follows :—As the ancestors 1of the Ascidiidae lost the power of reproducing by gem-mation, the vascular stolons became rudimentary, untilthey were useful merely as adhering organs. For sometime they would only be produced at the posterior endof the test (their original position in the Clavelinidaj),but in course of time they would extend further forwardsalong the left side of the body (the side upon which mostSimple Ascidians lie) so as to anchor the animal moresecurely, and we even find them occasionally in thiscondition in Ciona intestinalis and in Ascidia aspersa(Fig. 3)- They would then probably (in some not very remoteancest
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