EVOLUTION OF ORGANS IN THE CHORDATA. 273 



those of Selachians has been suggested by Huxley and P. 

 Fiirbringer, and is by Dohrn's results fully established. 

 Myxine is a further modification of Petromyzon, and shows a 

 remnant of the branchial skeleton in the cartilage of its ductus 

 cesophageo-cutaneus. The internal position of the branchial 

 artery in the embryo Petromyzon is simply explicable as a 

 consequence of the displacement of the branchial lamellae to- 

 wards the interior, and this change of position has been 

 brought about by the necessity of protecting the gills which 

 arose when the present habits of the animal (either burrowing 

 in mud or attaching itself to other animals) were acquired.^ 



Thus the theory that the branchial cartilages of Petromyzon 

 represent an archaic system not elsewhere present except in 

 the extra branchial cartilages of Selachians falls to the ground, 

 and with it disappear the consequences which Gegenbaur 

 formerly deduced from it. The Cyclostomata had no jaws it 

 was said because their ancestors had no true gill arches from 

 which jaws might be derived, whereas the truth is probably 

 they have lost the jaws through the conversion of the biting 



' In my paper on Myxine, in the previous number of this Journal, I have 

 described the habits of Myxine from actual observation. There can be no 

 doubt that during far the greater portion of its time the animal lies motion- 

 less, buried in mud, with only the extremity of its snout protruding. In this 

 condition the method of respiration, unique among fishes, namely, the constant 

 passage of a current of water through the nostril to the gill-pouches, is the 

 only method possible. Doubtless this method is also the most convenient 

 when the animal is boring into the body of a fish, or when its whole body has 

 penetrated into the flesh of its prey ; and it is difficult to say which of its 

 habits, burrowing or boring into its prey, was the prior cause in producing 

 the existing condition of the respiratory organs. I have not yet ascertained 

 whether the respiratory current is maintained by ciliary action, or by internal 

 muscular action, or by both combined. No muscular respiratory movements 

 are visible externally. Ammoccetes, it is true, burrows, although it has a 

 branchial skeleton ; and I do not know how the Ammocoetes, when buried, 

 can carry on the method of respiration which is seen in Petromyzon. Petro- 

 myzon never burrows, it conceals itself beneath stones and in crevices, but it 

 could not take in water by all its branchial apertures as it does unless it 

 were surrounded by water free from sediment. The comparison of the habits 

 of Petromyzon and Myxine illustrates the diversity of functions performed by 



