W. K. BROOKS ON THE GENUS SALPA. 199 



coetes as primitive, and as homologous with those which we find in the 

 jawed vertebrates ; and I have tried to trace the history of the modifica- 

 tions which have come between these structures of modern vertebrates 

 and those which we must attribute to the same organs at an earlier 

 genealogical stage in the primitive history of the ancestral pelagic chor- 

 data. The reader must judge of my success. 



Let us now see what light Dohrn's homology throws on the history 

 of these primitive modifications. He tells us (Studien, etc., VII, p. 47) 

 that he will point out, further on, the significance of the changes which 

 have led to the fusion, on the middle line, of structures which were 

 originally paired ; but I have been able to find nothing more upon this 

 point except the acknowledgment, on page 63, that "I frankly admit 

 that I have at present no available argument to bring the peculiar 

 organization (of the ciliated grooves) of ammoccetes from a pair of 

 imperforated (spiracular) gill-slits, into accordance with the concept of 

 change of function ; and that the origin of the slime-gland of ammo- 

 ccetes from two ventrally fused (mandibular) gill-slits must for the 

 present remain an unsolved problem." 



Whatever may be thought of my own view, it must be admitted that 

 Dohrn's homology of the endostylic system with two pairs of gill-slits has 

 very little phylogenetic value, even when measured by his own test : the 

 opportunity it furnishes for passing from the structure and functions of 

 modern organs to the history of earlier genealogical stages. 



Dohrn's memoirs upon the thyroid body are full of interesting 

 anatomical details, such as the similarity between the thyroid body of 

 the shark embryo and the true gill-slits, in their relations to the cartil- 

 ages, to the muscles and to the blood-supply (VII, p. 44) ; and the 

 resemblance between the peripharyngeal grooves of ammoccetes and 

 the spiracular gills of selachians (VIII, p. 55) ; but as he admits that 

 the annelidian hypothesis leaves the origin of the endostylic structures 

 of tunicates an unsolved problem, our subject, the history of the tuni- 

 cates, does not require us to enter into the discussion of these complicated 

 details of vertebrate morphology. 



The considerations which I have presented will undoubtedly be met 

 by the assertion that while the simple and direct origin of the tunicates 

 seems plausible so long as we confine ourselves to these animals alone, 

 such a restricted view is unscientific. I shall no doubt be told that we 

 are forced by more fundamental evidence to believe that the body cavity 

 of the chordata is, in ultimate analysis, a segmented enteroccel formed 



