GENESIS OF MAN. 49 



ment. The precise interval which they seem to have bridged over 

 lay between the Turbellaria and the Enter opnensta, the last of 

 which are represented by the well-known Balanoglossus. From 

 this point the great articulate branch swung off, and a little higher 

 the important branch of the Mollusks. 



To arrive at the eighth stage, we are again compelled to resort 

 to the fundamental biogenetic law, and reason from the chordonium 

 stage of embryonic development of all vertebrates to an extinct 

 form, which must have possessed the rudiment of a vertebral col- 

 umn in the form of a chorda dorsalis as a permanent character of 

 its adult state. It would have been wholly impossible to say 

 whether this assumed creature should be placed in the department 

 of articulates, mollusks or worms, were it not for the flood of light 

 which the anatomy of the Amphioxus and the Ascidian has, 

 within the past few years, shed upon the whole problem. 



The existence of such a chorda in the former of these animals, 

 and its presence also in the larval forms of the latter, are two facts 

 which point unmistakably to the type Vermes as the one which 

 has furnished the transition to the Vertebrata. No creatures have 

 been found in any of the other types which afford the least intima- 

 tion of any such transition, and neither in the Protozoa, the Zoo- 

 phytes, the Echinodermata, the Crustacea, the Arthropoda, nor the 

 Mollusca, has any trace of a chorda dorsalis, either in the larval or 

 adult state, been detected after the most thorough examination. 

 The conclusion is, therefore, irresistible, that the sub-kingdom 

 Vermes and the class Tunicata have furnished the true progenitor 

 of the vertebrates. This transition form itself has probably long 

 been extinct, but it has left lineal representatives in the Ascidia, 

 Phallusia, etc., which, while through long adaptation to a fixed 

 existence during their adult state they have lost their chorda, still 

 retain that distinctive character during their free larval state, as the 

 unquestionable ontogenetic representative of an organ which they 

 have inherited from their extinct chorda-bearing ancestor. This 

 ancient and primordial ancestor of the vertebrate sub-kingdom to 

 which so many facts, both of ontogenesis and of phylogenesis, with 

 so great certainty point, Haeckel denominates the Chordonium. As 

 if to put the solution of this important question beyond the possi- 

 bility of a future doubt, it is now found that a member of this same 

 group, the Appendicularia, actually preserves its chorda during life, 



