874 BTOLOaiCAL TRAINING AND STUDIES. 



rows of internally placed cells, and giving insertion by its sheath 

 to muscles. We have further the nervous system and the digestive 

 taking up in such embryos much the same positions relatively to 

 each other, and to this molluscan chorda dorsalis, that are taken up 

 by the confessedly homologous system in the Vertebrata ; we have 

 the nervous system originating in the same fashion, and closing up 

 like the vertebrate myelencephalon out of the early form of a 

 lamellar furrow into that of a closed tube ; we have, finally, the 

 respiratory and digestive inlets holding the vertebrate relationship 

 of continuity with, instead of the invertebrate of dislocation and 

 separation from, each other. Such are the facts ; but I am not 

 convinced that they will bear the interpretation that has been put 

 upon them ; though I must say the possession of this chorda dor- 

 salis by the active locomotor larva of the Ascidian which one day 

 settles down into such immobility lends not a little probability to 

 Mr. Herbert Spencer's view of the genesis of the segmented 

 vertebral column in animals undoubtedly vertebrate. But on this 

 view I should not be inconsistent with myself, inasmuch as, to waive 

 other considerations, the chorda dorsalis in each case would be con- 

 sidered as an adaptive or teleological modification, not a sign of 

 morphological kinship ^. Much perplexity may or must arise here ; 

 and whilst entertaining these views, I felt myself bound to examine 

 myself strictly to find whether in not taking them up, I might not 

 be giving way to that reactionary reluctance to accept new ideas 

 which advancmg years so frequently bring with them ; but a recent 

 paper, by Lacaze-Duthiers, published in the ' Comptes Rendus ' for 

 May 30, 1870, and translated in the 'Aunals and Magazine of 

 Natural History' for July 1870, would justify me, I think, in 

 calling that reluctance by another name. For in that paper the 

 renowned malaeologist just mentioned has brought to light the fact 

 that there is another sessile and solitary Ascidian, the Molgula 

 tuhulosa^ which goes through no such tadpole-like stage as had been 

 supposed to be gone through by all Ascidians except the Salpae, 

 which is never active and never puts out the activity which is so 

 remarkable in the other Ascidians, but settles down and remains 

 sedentary immediately after it is set free from the egg-capsule, 



^ See, however, Mr, Herbert Spencer's Appendix D to his principles of Biology, pt. 

 iv. chap. xvi. This appendix was printed in 1865, but not published till December 

 1869. I had not seen it when I wrote as above. 



