336 



Mr. E. Ray Lankester on the 



axis of growth (tertiary aggregates Fig. 



of Herbert Spencer) among Annu- 

 losa, and probably (tliough not 

 according to Spencer) among 

 Vertebrata, and even some Mol- 

 lusca — the process occurring at a 

 very early period and its results ■a'-\^:^\. 

 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 cp 

 aggregate develops a prostomium 

 or axis of anterior growth, the 

 chain necessarily breaks at that " 

 point [Microstomum, Tcenia^ Nai- 

 did^, Syllidse). The segmenta- 

 tion of the prostomial axis in ArdmcoJex (optical section) 

 Arthropoda and some Annelids, jw, prostomium ; pst, metasto- 

 wliich has an appearance of being 

 a zooid-segmentation comparable 

 to that of the metastomial axis, on 

 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 adaptational 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- 

 jihioxus. It is certain from those observations that the mouth 

 of Amphioxus is the first gill-slit or pharyngeal perforation 

 of the 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. 

 Amphioxus, then, and the Vertebrata have a new oral aperture, 

 the old one having been gradually suppressed. Comparative 



mmm; o, mouth; a, anus; s, 

 segmental or excretory aper- 

 ture ; ep, epiblast ; n, nerve- 

 centre ; rues, mesoblast ; hyp, 

 hypoblast. 



