384 



ZOOLOGY. 



The head of the sturgeon, garpifce, 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 chameleon they 

 ^^^— - are contained in special 



sacs (chromatophores) 

 which are under the 

 control of the nervous 

 system. 



The muscular system • 

 of Vertebrates arises 

 from the middle germ- 

 layer (mesoderm), and 



Fig. 385.— Placoid scale of dog-flsh (vertical sec- ;„ jt, ^ i-i „ ^„„„„-\ 



tion magnified), a, enamel layer ; b, dentine of the in me germ tne muSCleS 

 spine on the scale.-After Owen. jj^ p^j.j. anSC from the 



primary segments indicated by tlie protovertebrse, 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, vphich corre- 

 spond in number to 

 the vertebraa (Gegen- 

 baur). 



The eye in Verte- 

 brates in its develop- 

 mentalhistory belongs 

 to a different type of 

 structure from that of 

 any invertebrates, un 

 less it be the larval 

 Ascidians, for in both types the eye is said by Gegenbaur not 

 to be directly developed from the ectoderm, but from the 



Fig. 386.-Cyloid scale of roach, magnified, seen in 

 section, A, and from the surface, ^.— After Owen. 



