THE NATURALIST S OCCUPATION. 33 



and appearances would suggest relationship to the li- 

 chens sooner than to the vertebrates. If we cut open 

 the fleshy encrustation, and examine under the micro- 

 scope the contents of some of the little sacks found in 

 it, we discover some minute tadpole-like beings, repre- 

 sentinsf the larvae of the ascidian. The structure of 

 these remarkable creatures repeats the fundamental 

 features of the vertebrates so perfectly that we are com- 

 pelled to place them in the same great family. They 

 have a chordal axis with a nerve-tube on one side and 

 the alimentary tube on the opposite, with gill-slits per- 

 forating the throat, features common to all vertebrates 

 in early life. Although the adult ascidian bears not the 

 remotest resemblance to a vertebrate, the combination 

 of these characters in its larva proves that it belongs to 

 the vertebrate stock. The larva reaches the adult con- 

 dition by a process of degeneration. It fixes itself to a 

 stone by its head, then loses its tail, its only organ of 

 locomotion, and sinks into a purely vegetative existence. 

 So completely are its original features obhterated, that 

 its vertebrate nature would never have been suspected, 

 had not embryology brought to light its developmental 

 history. The striking agreement with the development 

 of the curious worm-like fish, Amphioxus, as made known 

 by Kowalevsky, a Russian embryologist, led Professor 

 Haeckel of Jena to regard the ascidian as the ancestor 

 of the vertebrate stock. Startling as such a proposition 

 was, it was favorably received at first, and was approved, 

 though with reserve, by no less a logical and critical 

 thinker than Huxley. Most authorities now concur with 

 Lankester and Dohrn in regarding both Amphioxus and 

 the ascidian as our degenerate vertebrate cousins. 



