558 



adult aseidian form meanwhile lias boon attained and the very 

 small individuals differ for the most part only in size from those 



It will be seen that some highly important features, recalling 



life of the embryo ascidian. Kowalevsky remarks that "the 



tion cells into the cells of the embryo, the formation of the seg- 

 mentation cavity, the conversion of this cavity into the body 

 cavity, and the formation of the digestive cavity through invag- 

 ination— these are all occurrences which are common to many 

 animals and have been observed in Amphioxus, Sagitta, Phoronis, 

 Echinus, etc. The first 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 



allying the Ascidians to the vertebrates, is the presence of a 

 ch'onh, dorsal 7.s, first seen in the adult Appendicular by J. 

 Muller. This organ is regarded by Kowalevsky to be functionally, 

 tis well as genetically, identical with that of Amphioxus. This 

 was a startling conclusion, and stimulated Professor Kupffer of 

 Kiel to study the ombrvofo- v of the ascidians anew. He did so, and 

 the results this careful observer obtained, led him to fully endorse 

 the conclusions reached by Kowalevsky, particularly those regard- 

 ing the unexpected relations of the ascidians to the vertebrates, 

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

 servers, as well as Metschnikoff, Ganin, Ussow and others, that the 

 vertebrates have probably descended from some type of worm re- 

 sembling larval ascidians more perhaps than any other vermian 

 type, though it is to be remembered that certain tailed larval Dis- 

 tomse appear to possess an organ resembling a chorda clorscdis, 

 and farther investigation on other types of worms may lead to 

 discoveries throwing more light on this intricate subject of the 

 ancestry of the vertebrates. At any rate, it is among the lower 



the vertebrates, as the Ccelenterates. Eehhmdenns, the Moilu-k-, 

 Crustacea and Insects, are too circumscribed and specialized 

 groups to afford any but characters of analogy* rather than affinity. 



