PISCES 199 



" The color of fishes is of threefold origin. The silvery luster is due to 

 crystals of guanin which occur in the skin. The other colors are due 

 partly to numerous strongly pigmented fat-cells and partly to the chro- 

 matophores in the derma, which under the control of the nervous system 

 can alter their forms and extent and thus produce color changes in the 

 fish, thus adapting it to its surroundings. It is of interest to note that 

 destruction of the eyes results in loss of power to change color." 1 



Locomotion and Appendages. The appendages of fishes, ex- 

 cept in rare instances, are the unpaired dorsal, anal, and caudal 

 fins, and the paired pectoral and ventral fins. "Fishes are 

 the only vertebrates having median fins supported by fin-rays." 

 The fin-rays supporting all the fins are of dermal origin. The 

 locomotion is mainly produced by the flexing of the body and 

 tail, so as to propel the usually spindle-shaped animal through 

 the water. The fins aid in directing the movements of the fish, 

 as does also the air-bladder, which regulates the specific gravity 

 of the fish. 



The skeleton is cartilaginous or bony. The notochord of the 

 protovertebrate becomes surrounded by a mesodermic sheath 

 which produces the centra of the vertebrae, consisting of cartilage 

 or bone. From the centra are outgrowths dorsally which give 

 rise to the neural tube, "an inverted tunnel of cartilage" en- 

 closing the cerebrospinal cavity, and ventral (hemal) outgrowths 

 protecting the viscera. Thelvertebrse are usually amphicelous, 

 and the notochord persists in the cavities between the centra. 

 The neural arches extend throughout the spinal column, while 

 the hemal are complete only in the tail. In the trunk the 

 hemal spines are absent and the hemal processes are divided 

 into basal processes and ribs which surround the viscera. There 

 is no sternum. 



The skull (Fig. 159) encloses the brain which does not fill the 

 cavity. The lower jaw is movable and usually bears teeth. 

 Some fishes have many teeth; others, few or none. They have 

 no other prehensile organs. 



The pectoral and ventral fins are homologous with the paired 

 limbs of the higher vertebrates, but lack many of the bones of the 

 higher forms, as a comparison of the bones of man's arm with 

 those of the pectoral fins will show. 



1 Hertwig's " Manual of Zoology," Kingsley, p. 559. 



