1895.] Dr Gaskell, The Origin of Vertebrates. 41 



function coexists, perhaps subordinately, with the primary func- 

 tion, or coexists in another member of the same group or in one 

 of its larval forms : the remarkable changes of function assumed 

 by Dr Gaskell had no such origin ; (iv) in any attempt at Phylo- 

 geny we must observe the " embryological unities " : Dr Gaskell 

 derives the epiblastic lining of the nervous system from the — in 

 the main — endodermic lining of the alimentary canal and the 

 mesoblastic " packing " cells around the vertebrate nerve-tube 

 — in part — from the endodermic "liver" of the Arthropod: (v) in 

 Phylogeny we must look at the most elementary of the animals 

 whose ancestor we seek, yet Dr Gaskell started some way up 

 the Vertebrate scale and omitted from his consideration the 

 Amphioxus and the larval Ascidian possibly because he regarded 

 them as degenerate but the same reasoning which led to a belief 

 in the degeneracy of these groups has been applied to shew that 

 the Lamprey is itself degenerate. 



The Arthropod characters of which we should expect to find 

 some trace in a Vertebrate descended from a Limulus-like ancestor 

 are (i) the ccelom, which in the Arthropod is rudimentary and 

 replaced by a body cavity of a highly specialized character and 

 yet the coelom reappears in the Vertebrate in the most primitive 

 form, just as it exists in the ancestor of the Arthropods them- 

 selves ; (ii) the chitinous skeleton which dominates the structure 

 of the Arthropods from one end of the group to the other, is un- 

 represented in their supposed descendants : in connection with 

 this it may be pointed out that cilia which with the exception 

 of the Nematodes, are universally present in all animals but the 

 Arthropods, have re-appeared in the Vertebrata ; (iii) the paired, 

 segmented limbs of Limulus, which are only represented by sensory 

 papillae round the mouth or by the branchiae 



The result of the whole hypothesis is that we are asked to 

 believe (i) that the functional alimentary canal of the Limulus, 

 with its — in the main — endodermic lining, has become the lining 

 of the Vertebrate nervous system which is epiblastic in origin ; 

 (ii) that the endodermic "liver" and the testes of the Arachnoid 

 ancestor have become the mesoblastic tissue surrounding the 

 nerve-cord of the Ammoccete ; (iii) that the Arachnoid ancestor 

 has developed an entirely new alimentary canal along its ventral 

 median line, and that this chitinous-lined tube is completed 

 ventrally by the fusion of the limbs ; (iv) that such of the seg- 

 mented limbs as are not required for this purpose become minute 

 sensory papillae surrounding the mouth ; (v) that the renal organs 

 of the Arachnoid ancestor degenerate iuto an appendage of the 

 brain, the pituitary body ; (vi) that part of the tissue representing 

 the testis of the ancestor is to be found inside the internal ear; 

 (vii) that the haemocoel of the Arthropod is again replaced by 



