RELATION OF ASCIDIANS TO VERTEBRATES. 30? 



point of difference from other animals in the development 

 of all vertebrates is seen in the formation of the dorsal 

 ridges, and their closing to form a nerve-canal. This mode 

 of formation of the nervous system is characteristic of the 

 vertebrates alone, except the Ascidians. Another primary 

 character allying the Ascidians to the vertebrates, is the 

 presence of a chorda dorsalis, first seen in the adult Appen- 

 dicularia by J. Miiller. This organ is regarded by Kowal- 

 evsky to be functionally, as well as genetically, identical with 

 that of Amphioxus. This was a startling conclusion, and 

 stimulated Professor Kupffer, of Kiel, to study the embry- 

 ology of the Ascidians anew. He did so, and the results this 

 careful observer obtained led him to fully endorse the con- 

 clusions reached by Kowalevsky, particularly those regarding 

 the unexpected relations of the Ascidians to the vertebrates, 

 and it would appear from the facts set forth by these emi- 

 nent observers, as well as Metschnikoff, Ganin, Ussow, and 

 others, that the vertebrates have probably descended from 

 some type of worm resembling larval Ascidians more perhaps 

 than any other vermian type, though it is to be remembered 

 that certain tailed larval Distomas appear to possess an organ 

 resembling a chorda dorsalis, and farther investigation on 

 other types of worms may lead to discoveries throwing more 

 light on this intricate subject of the ancestry of the verte- 

 brates. At any rate, it is among the lower worms, if any- 

 where, that we are to look for the ancestors of the Vertebrates, 

 as the Coelenterates, Echinoderms, the Mollusks, Crustacea 

 and Insects, are too circumscribed and specialized groups to 

 afford any but characters of analogy rather than affinity. 



For example, the cuttlefish, with its "bone,"' brain-cap- 

 sule and highly-developed eye, is, on the whole, more remote 

 from the lowest vertebrate, Amj)hioxus, than the Appendi- 

 cularia or the larval Ascidian. 



Certain (three) species of Molgula have been found by 

 Lacaze-Duthiers to have a nearly direct development, not 

 producing tailed young. There is a slight metamorphosis, 

 however, the young having five temporary, long, slender 

 processes. In Ascidia ampulloides the larva has a tail, no- 

 tochord and pigment spots, which are wanting in the young 



