PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 287 



to reproduce here the accessible material, aud wait for more iuformatiou 

 before considering the subject as fully decided. 



The figure here given, after Brandt's researches on AcanthocJiiton fas- 

 cicularis, may be supposed to present the general features of the nervous 

 system in the higher members of the group. 



The accompanying figure (C^) of part of the nervous system of Chiton 



Tt aLcVLvj dexrwoxv 

 cxn-fcxeu s 



Fig. C. — PBC, pedo-braucliial commissure ; NB, nervi brancliiales ; XP, nervi pedales ; 

 nl, nervi labialea, small filaments numerous and hardly traceable; ns, nervi pharyng. 

 superiores; (jpv, ganglia pedo-visceralia sen pedo-brancbialia; iapc, inter-anterio- 

 pliaryng. commissure ; ijypc, inter-]3edo-pharyngial commissure ; Ajyp, anterior interior 

 pliaryngial ganglia; aijyc, anterior inferior iiharjTigial commissure; j^Wj posterior 

 ditto; (/r, ganglia vascularia, resting on hv, a blood-vessel (the small commissure sep- 

 arating these ganglia is called by Brandt the intcrvascular commissure); sj), anterior 

 superior pliaryngial ganglia ; Psjy, posterior superior pharyngial ganglia ; x, superior 

 posterior post-pharyngial ganglion ; z, anterior superior i^haryugial commissure ; Isp, 

 inter superior pharyngial commissure ; oo, anterior inferior pharyngial nerves ; p]), 

 posterior ditto. 



termini as well as routes, with adults rather than embryos. We do not live in a world 

 of embryos alone, in any but the most metaphysical sense. We cannot learn the rela- 

 tions of animals, as they are, to each other from the embryological phylum alone, any 

 more than we could understand the nations of modern Europe and their political 

 boundaries from a map of the Aryan migrations. 



To apply this reasoning to the matter in hand in detail would reqiiire much more 

 space and time than are at present available. Yet it may be said that we have high 

 authority for considering that the mollnsks and worms are derived fi'om a common 

 origin, and that, in fact, the former derive their characteristic features from the ten- 

 dency to specialization and developement within the compass of a single segment, or a 

 very small number of segments, while the worms ai'c characterized rather by redupli- 

 cation of more simple segmental parts in great numb^r, but small variety among them- 

 selves. Various groups of mollusks may owe their greater or less i>articipation in fea- 



