1-1 



THE VOYAGE OF H.M.S. CHALLENGER. 



and Pachychlcena occupy the centre, may be placed dona, leading towards the Clave 

 1 in iil;c. while two genera, Corella and Corynascidia, occupy the opposite end. Abyss- 

 ascidia comes in between Ascidia and Corella, while the two somewhat aberrant forms, 

 Rhodosoma and Chelyosoma, must be considered as allied, the former to Ascidia and 

 Ciona, and the latter to Ascidia and Corella, but both having marked peculiarities of 

 their own, which prevent their being placed in the direct line between their allies. The 

 remaining form, Hypobythius, is in some respects (e.g., the structure of the branchial sac) 

 the most abnormal of all. It is allied to Ciona, and has also affinities with Ascidia or 

 Pachychlcena, but cannot be placed in a direct line between them. These relationships 

 may be shown in a schematic form thus : — 



Rhodosoma 



Ciona 



Ascidia 



Pachychlcena 



Chelyosoma 



■Abyssascidia 



Corella 



Corynascidia 



Hypobythivs 



This scheme might be divided by two vertical lines, so as to separate three groups, — - 

 a central, containing Ascidia and Pachychlcena, and two lateral, the one containing 

 Corella, Corynascidia, Chelyosoma, and Abyssascidia ; and the other the three remaining 

 genera, Ciona and the two abnormal forms Rhodosoma and Hypobythius. 



The table immediately following shows how these nine genera may be distinguished 

 by a few of their more important characters. It seems impossible, however, to arrange 

 them satisfactorily in sub-families. For example, the first division in this table, founded 

 on the condition of the dorsal lamina, throws Ciona in contact with Corella, and 

 separates it from the much more nearly allied Hypobyth his. In other respects, however, 

 this table is not an unnatural arrangement ; it brings Abyssascidia, Corynascidia, and 

 Corella into the same section, and puts Pachychlcena and Ascidia into close contact. 



