200 W. K. BROOKS. 



body, but with its oral end below, instead of above as it is in as- 

 cidian larvae. See, for example, DoHolum und sein Generations- 

 wechsel, by Grobbin. Plate IV., Fig. 19. 



Seeliger's carefully drawn figures of the organ of attachment of 

 the young clavelina (JeDaische Zeitschr., XVIII, 1885, Plate 4, 

 Figs. 40, 41, 42 and 44) show that it is at first represented by three 

 distinct and separate tracts of cylindrical ectodermal gland cells, 

 p. 36, ventral to the place of the future mouth, and it is only 

 necessary to read his minute account of its ontogeny, p. 36 and p. 52, 

 to see that its embryonic history is exactly what we should expect 

 it to be if it has been directly acquired as an organ of fixation. 

 If the phylogenetic history of the fixed ascidians has been as I 

 have pictured it, each successive stage in Seeliger's account of the 

 ontogeny of the organ of fixation is a recapitulation of a useful 

 stage in its ancestral history, and to my mind, furnishes conclu- 

 sive evidence that the ascidians are the descendants of swimming 

 tunicates. 



Willey's observations add nothing to Seeliger's excellent account 

 of the organ of fixation, and he gives no reason for holding that it 

 is a pre-oral lobe, except that " it contains loose mesenchyma cells 

 derived from the two lateral mesodermic bands." This is equally 

 true of other parts of the body cavity, and there is no more evidence 

 that the organ of fixation is a pre-oral lobe, than there is that it is 

 homologous with the jaws and teeth of sharks. 



If it is a pre-oral lobe it is a ventral one, and it cannot be com- 

 pared with the dorsal one of such protochordata as balauoglossus 

 and amphioxus. 



Willey's observation on the origin of the gill-slits of ciona are 

 most novel and interesting and they show that we may look for 

 most valuable results from the study of the subject in other tuni- 

 cates, but they are not suflficieut in themselves to prove that what 

 he has found in one species is typical for all. 



It seems to me that the author exaggerates our ignorance of the 

 subject, for while Seeliger's descriptive account of their origin in 

 pyrosoma is very short (Jenaische Zeitschr., XXII, 1888, p. 623), 

 his figures show that he has traced their history most minutely, 

 step by step, both in surface views and by sections, and that, in the 

 buds of pyrosoma, each gill-slit is an independent perforation, and 



