INTRODUCTION. 5 



being closely related to the section of the bivalves or 

 Lamellibranchiata. Kovvalevsky, however, discovered that 

 their embryonic development takes place on a plan so 

 similar to that of Amphioxus as almost to amount to an 

 identity. The development of the nervous and respiratory 

 systems, and of the axial skeleton or notochord in the 

 Ascidian embryo, as determined by Kowalevsky, showed 

 in the clearest manner that the relationship of the Ascidians 

 to Amphioxus, and through the latter to the Vertebrates, 

 was an extraordinarily close one. 



Kowalevsky's discovery of the chordate or sub-vertebrate 

 character of the Ascidian larva, was considered by Haeckel 

 as affording a direct solution of the problem of the con- 

 necting link between Vertebrates and Invertebrates. This 

 was a somewhat extreme view to take of the matter, since 

 Kowalevsky showed that the Ascidians could no longer be 

 regarded as true Invertebrates. 



In 1875 the foundation of the Annelid theory of 

 Vertebrate descent was laid independent!)' by Semper and 

 DoHRN ; and Kowalevsky's observations vi'ere explained 

 away in favour of the new line of speculation. It was the 

 discovery of the segmental origin of the excretory tubules 

 of the Selachian (shark) kidney, made independently and 

 simultaneously by Semper and Balfour, which may be 

 said to have led to the definite framing of the Annelid 

 theory. 



Dohrn approached the subject from a different point of 

 view. According to him, not only were the Vertebrates 

 not descended from forms allied to the Ascidians and 

 Amphioxus, but the latter were, by a process of almost 

 infinite degeneration, derived or degenerated from the 

 former. 



That the Ascidians are degenerate animals, to the 



