8 THE VOYAGE OF H.M.S. CHALLENGER. 



observations, especially upon Doliolvm and Append icularia. In America, Stimpson 

 described a number of new species, both from the American coasts and frum the seas of 

 China and Japan. Dr. J. D. Macdonald now commenced his valuable series of observa- 

 tions upon Tunicata, both pelagic and fixed. He described many new species from 

 different parts of the world, including some most remarkable forms, such as Chondro- 

 stachys, and Diplosoma. 



Bronn's " Thierreichs " (1852) contains the most complete revision of the entire group 

 which has yet been published. After a short history and a list of the principal works on 

 the subject, a complete and detailed account of the anatomy and embryology as known 

 at that time is given. This is followed by a tabular classification with diagnoses of the 

 orders, families, and genera ; the whole concluding with a section on the distribution of 

 the group. 



Grube's observations upon tbe Fauna of the Island Lussin, containing descriptions of 

 some new or imperfectly known Ascidians, and Lacaze-Duthiers' paper upon Chevreulius, 

 previously described as Rhodosoma by Ehrenberg in 1828, appeared a few years later. 



The following year (1866) was the date of the appearance of one of the most 

 important memoirs in the entire range of embryology, namely, Kowalevsky's "Entwicke- 

 lungsgeschichte der einfachen Ascidien." This was the first time that the Tunicata had 

 been treated according to modern embryological methods, and that the development of 

 the various organs had been worked out cell by cell, and their origin traced back to parti- 

 cular cell masses. Up to this jieriod all that was known of the process of development of 

 a Simple Ascidian was from Krohn's paper in 1852, on the embryology of Phallusia 

 mammillata, in which the various organs were described in the fully developed tailed 

 larva, but their process of development in the embryo was scarcely investigated. Hence 

 Kowalevsky worked upon comparatively fresh ground, and his remarkable memoir con- 

 tained results of the greatest novelty. He was the first to demonstrate the striking simi- 

 larity between the relations of the nervous system, the notochord, and the alimentary 

 canal in the larval Ascidian, on the one hand, and the vertebrate embryo on the other. 

 He also traced the development of the chief organs of the tailed larva from the segmented 

 ovum, and showed, in this case also, a certain similarity to the embryonic development 

 in a vertebrate. This pointed clearly to the fact that the Tunicata are closely allied 

 to the Vertebrata, and that the tailed larva represents the primitive or ancestral form from 

 which the adult Ascidian has degenerated ; and this led naturally to the view usually 

 accepted at the present day, that the group is a degenerated side branch from the lower 

 end of the vertebrate phylum. 



Kowalevsky's paper naturally drew other investigators into the same field. One of 

 the most important of these was Kupffer, who first of all took up the subject with the 

 view of, if possible, disproving Kowalevsky's results, but was speedily converted, and 

 soon became one of the strongest supporters of the new views. He published several 



