194 W. K. BROOKS. 



which have passed away ; to the study of the history of the modifi- 

 cations which have come between these structures and functions, 

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

 genealogical stage. 



I regard the structures which we find in the tunicates and in 

 ammocoetes as primitive, and as homologous with those which we 

 find in the jawed vertebrates ; and I have tried to trace the history 

 of the modifications 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 chordata. 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 ammocoetes from a paiy of imperforated (spiracular) gill-slits, 

 into accordance with the concept of change of function ; and that 

 the origin of the slime-gland of ammocoetes from two ventrally 

 fused (mandibular) gill-slits must for the present remain an un- 

 solved 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 phylogenetlc value, even when measui'cd 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, sucii as the similarity between the thyroid body 

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

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

 and the resemblance between the peripharyngeal grooves of am- 

 mocoetes 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, 



