336 



Mr. E. E,aj Lankester on tin 



axis of growth (tertiary aggregates 

 of Herbert Spencer) among Annii- 

 losa, and probably (thoiigb not 

 according to Spencer) among 

 Vertebrata, and even some Mol- 

 lusca — the process occm-ring at a 

 very early period and its results 

 being obscured, or even entirely 

 resolved, by later " integrating " 

 development in the two latter 

 cases — does not affect the prosto- 

 mium, which always has an axis 

 of anterior growth. When a 

 zooid-segment of a linear tertiary ep 

 develops a prostomium 



Fin 



-"^ 



chain necessarily breaks at that 

 point {Microstommn, Tcenia, Nai- 

 didge, Syllidaj). The segmenta- 

 tion of the prostomial axis in Archiscolex 



Arthropoda and some Annelids, 

 which has an appearance of being 

 a zooid-segmentation comparable 



(optical section) 



pr, prostomium ; pst, metasto- 

 mium ; o, mouth ; a, anus ; 5, 

 segmental or excretory aper- 

 ture ; ep, epiblast ; n, nerve- 

 centre ; mes, mesoblast ; hyp, 

 hypoblast. 



account of the identity in the 

 character of the appendages with 

 those of the metastomial axis, has yet to be explained. It 

 may be suggested that it is due to a distinct breaking up of 

 this axis like the posterior one into zooid-segments or zoon- 

 ites : there is much against this supposition (see Trans. Linn. 

 Soc. 1869, " On Chcetogaster and ^olosoma^^). Much more 

 likely, it seems, is the explanation that the oral aperture shifts 

 position, and that the ophthalmic segment alone in Arthro- 

 poda represents the prostomium, the antennary and antennular 

 segments being aboriginally metastomial and only prostomial 

 by later ada^ytational shifting of the oral aperture. 



The assumption of such a shifting of the oral aperture is 

 fully warranted by what has been demonstrated in the case 

 of Vertebrata through Kowalewsky's researches on Am- 

 j)hioxus. It is certain from those observations that the mouth 

 of Amphioxus is the first gill-slit or pharyngeal perforation 

 of tlie left side, and has no relation to a mouth such as that 

 which appears at an earlier phase of development in the allied 

 Ascidian larva, which latter mouth is that of Vermes generally. 

 Amfphioxus,ihexx, and the Vertebrata have a new oral aperture, 

 the old one having been gradually suppressed. Comparative 



