384 ZOOLOGY. 
The head of the sturgeon, garpike, and of other ganoid 
fishes, is protected by solid dermal bones, and the shells of 
turtles are dermal structures. 
The color of the skin of Vertebrates is due to pigment- 
granules situated either in the epidermis or dermis, and in 
the chameléon they 
are contained in special 
sacs (chromatophores) 
which are under the 
control of the nervous 
system. 
The muscular system 
of Vertebrates arises 
Ze from the middle germ_ 
oe layer (mesoderm), and 
Fig. 885.—Placoid scale of dog-fish (vertical sec- - 
tion magnified). @, enamel layer ; 0, dentine of the 1D the germ the muscles 
spine on the scale.—After Owen. _in part arise from the 
primary segments indicated by the protovertebre, while in 
the adults of fishes and certain salamanders, the muscular 
system is distinctly segmented, corresponding to the seg- 
mentation of the ver- 
tebral column, the 
four lateral trunk- 
muscles being divided 
into a number of seg- 
ments by tendinous 
bands, which corre- 
spond in number to 
the vertebree (Gegen- 
baur). 
The eye in Verte- 
brates in its develop- 
mental history belongs 
to a different type of 
structure from that of 
any invertebrates, un Fig. 386.—Cyloid scale of roach, magnified, seen in 
less it be the larval section, A, and from the surface, B.—After Owen. 
———— 
=u ge =. nS ii 
ata al A ae 
ft TT 
Ascidians, for in both types the eye is said by Gegenbaur not 
to be directly developed from the ectoderm, but from the 
