1884.] Aspects of the Body in Vertebrates and Arthropods. 859 
prolonged in transverse section, and the central canal which it 
contains also becomes vertically elongated.” Then follows the 
differentiation (1) of the epithelium of the central canal, (2) of the 
gray matter of the cord, and (3) of the internal coating of white 
matter. “The white matter is apparently the result of a differen- 
tiation of the outermost parts of the superficial cells of the cord 
into longitudinal nerve-fibers, which remain for a long period 
without a medullary sheath. * * * The gray matter and the 
central epithelium are formed by a differentiation of the main 
mass of the spinal cord.” 
There thus appears to be a lack of homology in the histologi- 
cal topography and origin of the nervous system in Chordata as 
compared with the annelidan worms and the arthropods, 
The relation of the nervous system of arthropods is constant ; 
after the stomodzum has been formed, commissures from the 
brain pass down and connect the latter with the subcesophageal 
ganglion, which is ventral. This relation of the postcesopha- 
geal nervous system to the ventral side of the body is as constant 
as the disposition of the ventral surface of the embryo of insects 
before the revolution a 
of the embryo, or of 
the embryos of anne- 
lid worms and Crus- _ € 
tacea. The position Ao 
of the arthropod em- ~~ 
bryo is the reverse of 
that of vertebrates. j Sw ho a = 4 
pi i |= an em (6 
Thevertebrate dispo” ota te pal anaE 
tive EDEK St E e We malphigian tubes; mesen, 
_ tive nervous system Sidintetine; At, heart; md, mandibles; mx, mz’, ISt 
is also seen in the and 2d maxilla. ’ From Ayers, with changes. 
embryo tunicate (Figs. 1, 2). 
Morphology —The brain of the Arthropoda is contained ina 
structure which throughout is lacking in homology with that of 
vertebrates. The crust, the segments, and the appendages espe- 
cially, have nothing in common with vertebrates, though the func- 
tions are, in a degree, the same. The origin and homologies of 
the sensory organs are, ad initio, different. For example, the eyes 
of arthropods are not truly homologous with those of vertebrates ; 
the cornea is simply a number of epithelial cells, while in verte- 
We BONES 
XA cP O00 O 
EEA ea 
` x 
= y 
d È) uy Qs 
a 
