STUDIES ON THE PEOTOCHORDATA. 337 



magnificeutly illustrated description, intended to prove this 

 point, which he gives in his thirteenth study (12) . 



From the latter memoir it appears that the spiracular 

 evagination of the pharyngeal wall on each side gradually 

 flattens itself completely out ; and then, at the place where it 

 formerly existed, a new groove (eine neue Vertiefung) arises, 

 namely, the so-called ^'Pseudo-branchialrinne." The latter is, 

 therefore, something new and distinct from the spiracular 

 evagination. The fact that it occurs at the spot formerly 

 occupied by the latter structure is rendered explicable by the 

 small space into which the various parts have been crowded in 

 the anterior region of Ammocoetes, so that the groove could 

 not form more anteriorly than the site of the rudimentary 

 spiracle, since the position of the latter is itself most anterior. 

 Apart, however, from such considerations as these, it is not 

 quite clear why the former existence of a pair of gill-slits 

 should be necessary for the formation of the so-called " Pseudo- 

 branchialrinne," and not for the formation of the other 

 exactly similar longitudinal grooves which occur dorsally, and 

 behind the duct of the thyroid ventrally, in the pharynx of 

 Ammocoetes. 



On the other hand, on account of its contiguity to the 

 " Pseudo-branchialrinne,"! should be emphatically inclined to 

 regard the anterior pair of rudimentary gill-slits of Ammo- 

 coetes, which according to Dohrn correspond to the spiracles 

 of Selachians, as homologous with the first pair of gill-slits of 

 the Ascidians and Amphioxus, especially since in the latter 

 there is the further point of resemblance to the condition in 

 Ammocoetes, that the first pair of gill-slits becomes oblite- 

 rated towards the end of the metamorphosis. In Ammocoetes, 

 as shown by Dohrn, these slits never break through, only the 

 endodermic pouches being temporarily formed. 



The homology of the endostyle and pericoronal groove of 

 Ascidians with the thyroid gland and " Pseudo-branchial- 

 rinne " of Ammocoetes was finally established by Dohrn, and I 

 believe, by a comparison of my observations on the Ascidians 

 and Amphioxus with those of Dohrn on Ammocoetes, that the 



