i 
860 Aspects of the Body in Vertebrates and Arthropods, [September, id 
brates the eye externally is an ingrowth of the epiblast. As the 
wings and legs of insects and organs of hearing and of smell are 
not the homologues of the parts which function as such in verte- 
brates, so we are not inclined to regard the heart and nervous — 
system of arthropods as truly homologous with the correspond- — 
ing organs of vertebrates. If there is such a fundamental difer — 
ence in the two types as regards the relations of the viscera to the — 
body-walls, and if this relation is common to all arthropods and 
the Annulata, we shall have to go back to the hypothetical — 
common ancestors of the tunicates and vertebrates on the one . 
hand and of the Annulata and Arthropoda on the other, for 
the means of comparison, It is not impossible that in animals 
allied to the planarian or nemertean worms, whose nervous sys- 
tem consists of a pair of dorsal ganglia, with two or more pais — 
of nerves passiag backward, that the common origin of the pr — 
chordate nervous system, and that peculiar to annelids and arthro- | 
pods, may yet be discovered. fe | 
ee ate oe 
So also the resemblance of the brain, dorsally situated, of the 
cephalopods, enclosed as it is in an imperfect cartilaginous @P 
sule, is interesting, but the relations are those of gael 
adaptation, and not of affinity. The mollusks, the annelids, 
arthropods and the vertebrates appear to be highly 
branches, and where there appear at first sight to be direct, cro 
homologies, so to speak, between them, these are rather pap 4 
dent structures, the result of adaptation rather than of w | 
descent. Examples of such, we believe, are the eye, the os 
and the heart of the cephalopods. a. rather it l 
The unity of organization in the animal world is seen origi" 
the homology of the cellular structure, and in the c 
of all from unicellular forms; and among the uar the 
identity of the morula and gastrula conditions, OF at origin it 
germ-layers ; and as regards the nervous system, 1N m à 
the epiblast, rather than in any special parts Or pa py the : 
highly elaborated and specialized types as are rep ‘ 
lobster, or butterfly, or fish. + followers 
The dispute between Cuvier and St. Hilaire and apse! sce 
was in part metaphysical. The old-time problems hee 
dental anatomy, such as comparing a lobster to a pee question 
its back, the problems of fore-and-aft symmetry, snd if we a 
of torsion in the fore and hind limbs of mammals, have 
