652 COMPARATIVE ANATOMY 



It asserts that the aUmentary canal is homologous in metazoa because it is 

 formed of endoderm. But there is no definition of endoderm except that 

 it is always the layer which forms the definitive alimentary canal. He 

 therefore concludes that the guts of vertebrates and Crustacea are not 

 homologous. 



There appear to be two fatal objections to the Gaskell arachnid hypo- 

 thesis of vertebrate ancestry. First, the theory ignores the evidence of 

 the homology of the germ layers, by deriving the neural tube from the 

 alimentary canal. Second, the theory leaves a heart between the pharynx 

 and the nervous system, so that in order to meet this difficulty it is 

 necessary to assume that the crustacean heart disappears and that a new 

 one takes its place. There is, however, no embryological evidence to 

 support this assumption. 



It may be further urged against the theory that it pictures the 

 ependyma of the vertebrate neural tube as receiving cellular increments 

 from adjacent tissue to form the nervous tissue of the brain and cord. 

 In ontogenesis, however, the mantle and marginal layers of the neural 

 tube are not formed by additions from without, but by the proliferation of 

 cells of the ependymal layer itself. 



Of Gaskell's theory. Patten, who has an arachnid theory of his own, 

 says: "Gaskell at least makes a valiant fight to save the pieces of the 

 invertebrate nervous system, even if he does annihilate the rest of the 

 animal in the attempt." 



Patten's Arachnid Hypothesis 



According to Patten (191 2), the structural plan of an arachnid such 

 as limulus and a primitive vertebrate such as an ostracoderm is "all the 

 same," but the arachnid head and body correspond to the vertebrate head 

 only. Nearly all of the vertebrate body consists of a new generation of 

 metameres, not represented in arachnids. (Figs. 10, 17) 



Patten, although agreeing with Gaskell that the vertebrate mouth is 

 new, differs in his explanation of the closure of the old mouth. In Patten's 

 view, the closure of the old mouth is due to the "conditions created by 

 apical growth, by cephalization, and by the increase in the volume of the 

 yolk sphere." The new mouth is derived from an ancient arthropod organ 

 known as the "dorsal organ," which in limulus becomes the "cephalic 

 navel." What its original significance may be is not apparent. 



Patten sees no difficulty in converting the nerve cord of arachnids into 

 the tubular cord of vertebrates. In the brains of both he finds the same 

 five divisions. The resemblance holds good even for their subdivisions 

 and their histological details. He lays much stress upon the numerical 

 correspondence between the neuromeric divisions of the brain described 

 by Locy and Hill and those which he finds in limulus. There are, he 



