Prof. C. Claus's Reply to Prof. Lanhesters '* Rejoinder. ^^ 469 



waste no more -words ; but the charge which is cast upon me 

 hj the ingenious author of the hypothesis " of the change of 

 position of the buccal aperture in the Arthropocla," of having 

 designedly ignored his memoir " On the Primitive Cell-layers 

 &c." of the year 1873, in order to cite as my own (in my 

 memoir on the Daphnidce, 1876) his discovery tliat the 

 Arthropod-antennee are appendages of the trunk, is one to 

 which I cannot avoid offering the full tribute of my admira- 

 tion, and I am only in doubt whether I should wonder more 

 at the acuteness of its logic or at the delicacy of its sentiment. 

 Does Ray Lankester seriously believe tliat by this hypothesis 

 of his, viz. the displacement of the mouth * in the Arthropoda 

 from the false analogy of Amphioxus^ he can establish in the 

 mind of the judicious reader even the shadow of a claim to the 

 interpretation and demonstration of the second antenna of the 

 Crustacea as a trunk-appendage ? Does the " adaptational 

 shifting of the oral aperture," by which the two anterior pairs 



* I here cite the whole passapfe relating to the hypothesis of the dis- 



Elacement of the mouth, iu order that I may in no respect lessen Ray 

 lankester's services in the eyes of the reader of my reply (see Ann. & 

 Mag. Nat. Hist. ser. 4, vol. xi. p. 335) : — '^ The prostomium in Triplo- 

 blastica is liable to be suppressed altogether in the course of individual 

 development, the mouth becoming terminal or other modifications arising ; 

 but wliere it does appear it constantly carries the chief organ of sight, 

 whilst the auditory sac is prostomial in Turbellariaus, but metastomial 

 in Tunicates, Vertebrates, and Mollusca. 



" The production of individuals of an increased complexity of organiza- 

 tion among Triploblastica, by the linear aggregation of zooids, produced 

 by budding in the posterior or metastomial axis of growth (tertiary aggre- 

 gates of Herbert Spencer) among Annulosa, and probably (though not 

 according to Spencer) among Vertebrata, and even some Mollusca — the 

 process occurring 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 prostomium, which always has an axis 

 of anterior growth. When a zooid-segment of a linear tertiary aggregate 

 develops a prostomium or axis of anterior growth, the chain necessarily 

 breaks at that point {Microstomum, Tcenia, NaididiB, Syllidae). The 

 segmentation of the prostomial axis in Arthropoda and some Annelids, 

 which 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 ex- 

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

 this axis like the posterior one into zooid-segments or zoonites : there 

 is much against this supposition (see Trans. Linn. Soc._ 1869, ' On Chato- 

 ff aster and ^olosoma'). Much more likely, it seems, is the explanation 

 ithat the oral aperture shifts position, and that the ophthalmic segment 

 alone in Arthropoda represents the prostomium, the autennary and anten- 

 nular segments being aboriginally metastomial and only prostomial by 

 later adaptational shifting of the oral aperture.''^ And tlien follows, for 

 the complete confirmation 'of this " adaptational shifting." the passage as 

 to the mouth of Amphioxus, already cited in my previous article (July 

 1886, p. 62). 



Ann. (& Mag. N. Hist. Ser. 5. Vol. xviii. 32 



