No. 470] UNITY OF GNATHOSTOME TYPE 



stomes. It is a single patch of cells symmetrically placed with 

 reference to the sagittal plane of the body. In reality it is a double 

 patch, although the indifferent tissue, which later forms a septum, 

 has not at this stage developed. 



It is said that in Acipenser embryos a median unpaired nasal 

 plate precedes the paired nasal organ. This being the case, we 

 have all the more reason to consider the IMarsipobranch nose a 

 paired structure, even though it appears to develop from an 

 unpaired plate. 



There is certainly no truth in Haeckel's dictum that "the 

 pharyngo-nasal canal [of ]\Iyxinoids] is a secondary acquirement 

 in connection with parasitic habit." 



Notwithstanding that much has been said about the naso- 

 pharyngeal tube of the ^Nlyxiuoids, the full significance of this 

 structure has not yet been made out. While It at first seemed to 

 be an organ at the height of its devcl(>))ni(Mit in H(h'lh).st()inii, the 

 embryological evidence would indicate th;U it is a vcrv old >\r\u-- 

 ture.' We should not forget that in the rctn.niv/niii, > it is 

 already closed off from the pharynx, itiid that in all otiirr \rv\r- 

 brates it arises in a very early stage of ciiibryonic life a- a na>o- 

 hypophysial invagination of the ectoderm to\\artl> the nu ^mtcroii. 

 It would thus seem to be on the verge of e.xtinctiini in the ( vclo- 

 stomes. No other stages of its development are known to us. 



The total absence of an auditory organ in ampliioxus is held 

 by certain zoologists to be a difficulty in the way of accepting this 

 animal as an ancestor of the vertebrate stock. 



They point out that in the tunicates, esi)ecially in the A|)})endi- 

 cularia, there is an otoeyst with iiielos(>(l otolith, wliich >n|)- 

 posedly serves as an organ for the |>ereej)tioii of \\a\e motion in 



relationship to the v<M'tel>rate stock, it is very unlikely that any 

 form int(M-me.iial(> between them and the vertel.rates would 

 entireh hu-k an anditorv ot-an The rvv»v in their nNiMMung 

 hes in the fa<t that \hv. a^^nne that the aiidilMn wi-an. of the 



Thi^ i. not the ea.e. a^ all the exidenee, l.oth nioiphologic and^ 



a neomorph arising within the vertel)rate stock, from a sense- 



