336 AUTHUl? WTLLEY. 



We have already seen, especially with the reference to the 

 mode of origin of the branchial stigmata in the two cases, that 

 on the whole the development of Ciona, as well as that of the 

 Ascidise proper, presents obviously much more primitive 

 features than that of Clavelina, which was the object studied 

 chiefly by van Beueden and Julin. It may, therefore, be stated 

 generally that the stigmata in the branchial sac of the 

 Ascidians are derived from and represent three pairs 

 of primary gill-clefts. 



The independence of the metamerism of the gill-slits from 

 that of the trunk as a whole is shown in the plainest possible 

 way in Balanoglossus. 



In consequence of the view, based on embryological data, 

 which was put forward in 1873 by Wilhelm Mliller (30), as to 

 the homology of the thyroid gland of Ammocoetes with the 

 endostyle of the Ascidians and Amphioxus, and which was 

 confirmed and in fact definitely demonstrated by Dohrn in 

 1885 (10), who also first pointed out the homology of the 

 pericoronal ciliated groove of Ascidians (sillon pericoronal — 

 Julin) with the so-called " Pseudo-branchialrinne" of Ammo- 

 ccetes, it is necessary to say a few words about Dohrn's con- 

 ception of the morphological value of the latter structure. 



I may first, however, point out that I have on a former 

 occasion (33, p. 209, and plates) figured and described a cor- 

 responding peripharyngeal ridge in Amphioxus. 



Dohrn says (11, p. 310), "Die Pseudo-branchialrinne [of 

 Ammocoetes] geht factisch hervor aus der nicht mehr zum 

 Durchbruch gelangenden beiderseitigen vordersten Entoder- 

 mausstiilpung des Ammocoetes, welche [on account of the 

 relations to it of the Trigeminus] der Spritzlochspalte der 

 Selachier und Ganoiden, der Pseudo-branchie der Teleostier 

 homologisirt werden muss." Although Dohrn may be said to 

 have demonstrated the homology of the so-called " Pseudo- 

 branchialrinne " of Ammocoetes with the pericoronal groove of 

 the Ascidians, yet he was far from proving the identity of the 

 former with the rudimentary spiracle of Ammocoetes, as 

 appears at once from a careful perusal of the detailed and 



