46 Br Oaskell, The Origin of Vertebrates. [Dec. 2, 



is based on Macleod's observations, and is considered reasonable 

 and indeed highly probable by all comparative anatomists who 

 know anything about the subject. 



There is on my theory no need to assume any sudden change 

 of function or of anatomical arrangement ; I do not presuppose 

 any transformation anything like as sudden or as marvellous as 

 that which takes place under our eyes at the time of transfor- 

 mation, when the Ammoccetes becomes a Petromyzon. 



Mr Shipley's difficulties about the absence of cilia and the 

 presence of chitin in Arthropods do not appear serious, seeing that 

 the alimentary canal of Daphnia appears to be ciliated according 

 to Hardy's paper and also the laminated layer of the skin of 

 Ammoccetes is histologically exactly the same as an internal 

 chitinous layer. Finally, in answer to the argument of Mr 

 Sedgwick and Mr Shipley that Ammoccetes is not the lowest 

 Vertebrate, that therefore I ought to have compared the Limulus 

 and Scorpion forms with Amphioxus and the Tunicates, to which 

 Mr Shipley was careful to add that, if I answered that these were 

 degenerate vertebrate forms, then he had as good right to call the 

 lamprey degenerate too. I wish to point out that the whole trans- 

 formation of the Ammoccetes to the Petromyzon is characterised 

 by the discarding and throwing away of invertebrate character- 

 istics and the putting on of a more distinctly vertebrate appear- 

 ance. 



We see how by a process of histolysis, the tubular muscles, the 

 muco-cartilage, the thyroid gland, the velar, tentacular and 

 branchial appendages are all destroyed, or utterly changed and 

 new tissues are formed, the net result of which is not degeneration 

 but the formation of an animal of a distinctly higher grade, 

 distinctly more comparable with other vertebrates, as instanced by 

 the possibility of a direct comparison between the skull of 

 Petromyzon and that of a tadpole, by the growth of the brain and 

 the appearance of new nerves such as the hypoglossal, the coming 

 out of the eyes, and the free swimming fish-like form. In fact 

 just as the comparison between the Scorpion-like ancestor and the 

 Vertebrate is shown in the organs of the larval stage, viz. 

 in Ammoccetes, so the comparison between the higher Vertebrates 

 and Ammoccetes is shown by the organs of the adult stage, viz. 

 Petromyzon. I utterly fail to see any reason for considering the 

 lamprey as degenerate, and suppose that Mr Shipley is referring 

 to Dohrn's overstrained and untenable hypothesis that the lamprey 

 is a degenerate Elasmobranch. 



On the other hand the nearest allies to the Amphioxus 

 and the Tunicates are the very Ammoccetes themselves, but 

 how transformed ! in the case of the former, no eyes, no cerebral 

 hemispheres, no special organs of sense; while in the latter, with 



