APPENDIX D. 



ON THE ORIGIN OF THE VERTEBRATE TYPE. 



[ When studying the development of the vertebrate skeleton, there 

 occurred to me the following idea respecting the possible origin of the 

 notochord. I was eventually led to omit the few pages of Appendix in 

 which I had expressed this idea, because it was unsupported by develop- 

 mental evidence. The developmental evidence recently discovered, how- 

 ever, has led Professor Haeckeland others to analogous views respecting 

 the affiliation of the Vertebrata on the Molluscoida. Having fortu- 

 nately preserved a proof of the suppressed pages, I am able now to 

 add them. With the omission of a superfluous paragraph, they are 

 reprinted verbatim from this proof, which dates back to the autumn 

 of 1865, at which time the chapter on " The Shapes of Vertebrate 

 Skeletons" was written. December, 1869.] 



The general argument contained in Chap. XVI. of Part IV., I 

 have thought it undesirable to implicate with any conception more 

 speculative than those essential to it ; and to avoid so implicating 

 it, I transfer to this place an hypothesis respecting the derivation 

 of the rudimentary vertebrate structure, which appears to me 

 worth considering. 



Among those molluscoid animals with which the lowest verte- 

 brate animal has sundry traits in common, it very generally happens 

 that while the adult is stationary the larva is locomotive. The 

 locomotion of the larva is effected by the undulations of a tail. In 

 shape and movement one of these young Ascidians is not altogether 

 unlike a Tadpole. And as the tail of the Tadpole disappears 

 when its function comes to be fulfilled by limbs ; so the Ascidian 

 larva's tail disappears when fixation of the larva renders it useless. 

 This disappearance of the tail, however, is not without exception. 

 The Appendicularia is an Ascidian which retains its tail through- 

 out life ; and by its aid continues throughout life to swim about. 

 Now this tail of the Appendicularia has a very suggestive structure. 

 It is long, tapering to a point, and flattened. From end to end 

 there runs a mid-rib, which appears to be an imbedded gelatinous 

 rod, not unlike a notochord. Extending along the two sides of 



