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Brain in Vertebrates and Invertebrates. 307 
value in determining the homology of its aspects than are the 
relative positions of the nervous and vascular centres to a 
given surface or aspect of the body. I concluded, therefore, 
that the “ neural surface”’ of an Arthropod, indicated by the 
position and course of the main or central part of the nervous 
system, was homologous with the surface or aspect of the body 
so indicated in a Vertebrate; also that the condition which 
seemed to alter that relation in respect of a small part of the 
neural axis, in the Invertebrate, was inadequate to affect con- 
clusions of homology based on the permanent relations to sur- 
faces of the body shown by the nervous and vascular centres 
respectively, especially as such condition, affecting a small part 
of the Invertebrate nervous system, was discernible, though 
transitory, in the brain of the Vertebrate embryo. 
Consequently I proposed to apply the term “ neural” to 
the surface or aspect of the body which in progressive motion 
is carried upright and directed backward in man, and to 
apply the term “‘hemal” to the surface or aspect of the 
body which is carried upright and turned forward in man; 
and I proposed to apply the same terms to signify homologous 
surfaces indicated by the positions of the nervous and vascular 
centres, which surfaces are horizontal and are carried respec- 
tively upwards and downwards in Vertebrates below mankind, 
but are carried downwards and upwards respectively in the 
Invertebrates above the “ Radiaires”’ of Cuvier. 
In higher members of this division (the starfish for ex- 
ample) the homologue of the “ cesophageal ring ”’ is the centre 
of the nervous system. As organization progresses the por- 
tion of the ring opposite to the part or aspect on which the 
mouth opens becomes the seat of developments in relation to 
the necessity of a provision receptive of impressions or influ- 
ences affecting the more exposed and upturned surface of the 
body. Next follows the prolongation of the post-oral part of 
the frame, which may be segmented and cuirassed in some or 
be left sott and unjointed in others. But the primitive neural 
ring round the gullet remains in both the Annulate and Mol- 
luscous series. Such course of the gullet is finally checked at 
the higher Vertebrate stage by the development of a branchial 
chamber which takes in the materials for digestion as well as 
for respiration. 
