xii.] THE GENEALOGY OF ANIMALS. 315 



principal stirpes, of which one, a, became the root of the 

 Annelida, Echinodermata, and Arthropoda, while the 

 other, b, gave rise to the Polyzoa and Ascidioida, and 

 produced the two remaining stirpes of the Vertebrata 

 and the Mollusca. 



Perhaps the most startling proposition of all those 

 which Professor Haeckel puts before us is that which 

 he bases upon Kowalewsky's researches into the deve- 

 lopment of Amphioxus and of the Ascidioida, that the 

 origin of the Vertebrata is to be sought in an Ascidioid 

 form. Goodsir long ago insisted upon the resemblance 

 between Amphioxus and the Ascidians ; but the notion 

 of a genetic connection between the two, and especially 

 the identification of the notochord of the Vertebrate 

 with the axis of the caudal appendage of the larva of 

 the Ascidian, is a novelty which, at first, takes one's 

 breath away. I must confess, however, that the more 

 I have pondered over it, the more grounds appear in 

 its favour, though 1 am not convinced that there is any 

 real parallelism between the mode of development of 

 the ganglion of the Ascidmn and that of the Vertebrate. 

 cerebro-spinal axis. 



The hardly less startling hypothesis that the Echino- 

 derms are coalesced worms, on the other hand, appears 

 to be open to serious objection. As a matter of anatomy, 

 it does not seem to me to correspond with fact ; for there 

 is no worm with a calcareous skeleton, nor any which 

 has a band-like ventral nerve, superficial to which lies 

 an ambulacral vessel And, -as a question of develop- 

 ment, the formation of the radiate Echinoderm within 

 its vermiform larva seems to me to be analogous to the 

 formation of a radiate Medusa upon a Hydrozoic stock 

 But a Medusa is surely not the result of the coalescence 

 of as many organisms as it presents morphological 

 segments. 



