328 ARTHUR WILLEY. 



as to whether its primary position has a phylogenetic signi- 

 ficance. The remarkable constancy of the latter and its 

 analogy with Amphioxus would seem to indicate that it has. 



Professor Brooks says (p. 200), " Willey's observations 

 add nothing to Seeliger's excellent account of the organ of 

 fixation" (except to show that it behaves very differently in 

 Ciona from what it does in Clavelina, the differences being of 

 such a nature as to affect very sensibly the morphological in- 

 terpretation of the structure) ; " and he gives no reason for 

 holding that it is a pre-oral lobe, except that it contains loose 

 mesenchyme-cells derived from the two lateral mesodermic 

 bands." This is a complete misrepresentation. What I chiefly 

 relied on in forming my opinion as to its morphological value 

 was its topographical relations. It is the barest statement of 

 the facts of the case to say that it is a lobe, that it is pre-oral, 

 and that its cavity is the anterior and pre-oral portion of the 

 body-cavity. Under these circumstances I confess my in- 

 ability to understand how the suggestion as to the possibility 

 of this pre-oral lobe being genetically related to a similarly 

 placed structure in Amphioxus can, on any pretence, be re- 

 garded as not being entitled to the least consideration. The 

 presence of loose mesenchyme-cells in place of a lining epi- 

 thelium was emphasised by me as a necessary evil common to 

 the rest of the body-cavity. Professor Brooks, however, 

 argues as follows: — ''This (i.e. the presence of loose mesen- 

 chyme-cells) is equally true of other parts of the body-cavity, 

 and there is no more evidence that the organ of fixation is a 

 pre-oral lobe than there is that it is homologous with the jaws 

 and teeth of sharks." 



" How much,'^ I ask — '' how much consideration is this 

 argument entitled to ? " 



" If," continues Professor Brooks, " it is a pre-oral lobe, it 

 is a ventral one, and it cannot be compared with the dorsal one 

 of such protochordata as Balanoglossus and Amphioxus.'^ I 

 venture to think that the reflections urged in the foregoing 

 contribution. No. Ill, will demonstrate that here Professor 

 Brooks has fallen into an egregious error. The primary 



