238 AETHUR WILLEY. 



of the trunk. Not only does a portion of the gut hecome pro- 

 jected into the proboscis^ with the result that its lumen has 

 become vestigial and its walls rigid, but gill-slits have been 

 abolished from the anterior portion of the gut which lies in the 

 collar region.^ Masterman's pleurochords lie in the collar or 

 lophophoral region, and from his writings ^ they appear to be 

 vestiges of the gill-clefts which still persist in Cephalo- 

 discus. 



As we have seen, the stomochord of the Enteropneusta is 

 a derivative of the collar-gut, and retains vestiges of structures 

 formerly serving another function in the post-oral collar region. 

 Thus we may conclude, in accordance with the preceding 

 considerations, that the pleurochords of Actinotrocha, the 

 gill-clefts of Cephalodiscus, and the lateral pouches of the 

 enteropneustic stomochord are the persistent vestiges of primi- 

 tive gill-clefts belonging to that portion of the body which, in 

 the Enteropneusta, is now specialised as the collar region. 

 The great series of truncal gill-clefts is entirely lacking in 

 the sessile forms, just as in the Ascidians there are strong 

 grounds for the interpretation of the numerous branchial stig- 

 mata as having originated by the subdivision of a single pair of 

 gill-slits^ which persist in their undivided condition in Appen- 

 dicularia.* 



' 111 Amphioxus the first larval gill-slit closes up. 



^ A. T. Masterman, "On the Further Auatoniy and the Budding Processes 

 of Cephalodiscus dodecalophus," 'Trans. Roy. Soc. Ediu.,' vol. xxxix, 

 1898, p. 507. 



' From the mode of origin of the i)rimar.y branchial stigmata in Ciona 

 intestinalis I thought three primary gill-clefts were represented in the 

 Ascidians, but a study of their formation in Molgula manhattensis con- 

 vinced me that such an interpretation could not be upheld ; and on this point 

 I modified my views, and am now disposed to recognise the truth of Van 

 Beueden and Julia's hypothesis as to the presence of one pair only of primary 

 slits (see A. Willey, ' Amphioxus and tiie Ancestry of the Vertebrates,' 1894, 

 p. 232). 



* It follows from what has gone before that the anterior portion of the 

 body of the stomocliord in Enteropneusta, that is the part intervening be- 

 tween the vermiform process (vvlicn present) and the rej;ion of the ctecal 

 pouches (Fig. 3, a. «.), corresponds to the functional ceso|)hagus of Actino- 

 trocha ; not that Actinotrocha is itself an ancestral form, but it appears to 



