REPORT ON THE TUNICATA. 



139 



that they are really rudimentary stolons, which are, however, not capable of producing 

 buds, and are only made use of as adhering organs/ 



In most of the other members of the family Ascidiidiv), these rudimentary stolons 

 either become lost altogether or they acquire a new function, that of aiding in respira- 

 tion, and become converted into an important system of blood-vessels ramifying 

 through the test and terminating in numerous enlarged bulbs in its superficial layer 

 (Fig. 25). 



Fia. 25.— Diagram of the vessels iu the test of Ascidia mammillata. 



B. a small part of the system more highly magnified; a. v. afferent vessels; e.v. efferent vessels; i.k. terminal 



knobs ; (. sxirface of the test. 



This is a well-marked instance of the evolution of a system of organs performing an 

 important function from a structure having originally an entirely different function. 

 The process has, I believe, been as follows : — The bud-producing stolons of the Clave- 

 linidse lost their power of gemmation in the primitive Ascidiidte, and the stolons became 

 rudimentary. In Cmia and some other Simple Ascidiidse they were made use of 

 as adhering organs, until they gradually came to function slightly as respiratory organs 

 by aiding in the oxygenation of the blood circulating in the test, and were then seized 

 upon by natural selection as useful organs, and evolved into the system of vessels seen 

 iu Ascidia mammillata and other species. It would be possil;le to form a series of 

 preparations showing all the transition forms between the bud-producing stolons of 

 Clavelina (Fig. 19, p. 129), and the respiratory system of vessels in the test oi Ascidia 

 (Fig. 25) ; and Ciona (Fig. 24) may be regarded as the nearest form known to the 

 point occupied by the ancestors in which the change of function took place. 



1 See Prelim. Report on Chall. Tun., Part II., Proc. Roy. Soc. Edin., vol. x. p. 719, 1880 ; and On the Evolution 



of the Blood-vessels of the Test in the Tiinicata, Xaiurc, vol. xxxi. p. 247, 1885. 



