940 The American Naturalist. [November, 
geal commissures of the Articulates. Can this supposition be 
upheld? I do not think that the invagination of the optic 
vesicles in Vertebrates will be said by any morphologist to 
invalidate the comparison. At first it seems to reduce the so- 
called “ brain” of Articulates to insignificance, but as we know 
that this brain, or more correctly, supra-oesophageal ganglion, 
is mainly a visual ganglion in Articulates, and it is by no 
means a great degradation for it to appear merely as a retina 
in Vertebrates. We must not be influenced by the misuse of 
the word “brain ” applied to Invertebrates. But how can two 
diverging vertebrate eyes, which form a Y with the main ner- 
vous system, be the same as a complete nervous ring around 
the oesophagus? This difficulty is met by assuming that the 
eyes and optic ganglia fail to meet in the median line in front. 
In connection with this assumption, remarks upon two points 
are necessary. First, in Invertebrates with oesophageal nerve 
rings, the supra-oesophageal ganglia do not apparently arise in 
the median line, but as two symmetrical anlages, which sub- 
sequently meet in the median line. I recall that this is the 
method of development in Lumbricus (E. B. Wilson), Arach- 
nids and Insects (Patten), Crustacea (Kingsley), and in Crepi- 
dula (Conklin). Other observations could be cited.” Hence 
we have only to believe that in the Vertebrates the separation 
is maintained and increased. Second, there should be found a 
cause for the separation of the eyes (supra-oesophageal gang- 
lia) in the Vertebrates. This is given by the growth of the 
vertebrate brain, which is enormous and precocious. If the 
brain acquires great size, the eyes are correspondingly sepa- 
rated. It is evident that our assumption makes the brain 
homologous with the suboseophageal ganglia probably plus 
several ganglia of the ventral chain, the remainder of the ven- 
tral chain becoming homologous with the spinal cord. 
Next as to the invertebrate mouth. If the eyes and optic 
nerves represent the oral nerve ring, we must look for the 1m- 
vertebrate mouth between the eyes. The invertebrate mouth 
3? That the observations are conflicting as to the paired origin of the Articul ya 
brain is well known, but the theory that the median Scheitelplatte evolved into 
the Articulate brain encounters difficulties which seem to me very serious. 
