JV. VERTEBRATA: PISCES. 559 



Spines (strongly developed single scales) and larger bony plates, 

 these last usually resulting from the fusion of numerous scales. 



The coloration of fishes is threefold in origin. The silvery lustre is 

 due to crystals of guanin which occur not only in the skin but in the peri- 

 toneum and pericardial walls. In some fishes from their iridescence 

 (AWurnus lucidus) these crystals become of commercial value. They are 

 freed from the skin by boiling with ammonia and, suspended in the fluid, 

 form the important part of essence of pearl (essence d'orienl) which is 

 used in making artificial pearls, being either applied to the outside of ala- 

 baster balls (Roman pearls) or as a coating to the inside of glass beads 

 (Paris pearls). The other colors of fishes are due in part to the numerous 

 strongly pigmented fat cells, in part to ' chromatophores' in the derma, 

 which, under control of the nervous system, can alter their form and 

 extent and thus produce color changes in the fish. It is by means of 

 these chromatophores tiiat fishes adaiit themselves to their surroundings. 

 It is of interest to note that destruction of the eyes results in loss of power 

 to change color. 



The axial skeleton shows many conditions which are unknown 

 outside the class, and varies in character from group to group, the 

 most important differences consisting in its cartilaginous or bony 

 character. The vertebrae are nearly always amphicoelous, the 

 notocliord persisting in the cavities between the successive centra 

 (fig. 557). iSTeural and luemal arches occur, these having as key- 

 stones the unpaired s2)inous ])rocesses. The neural arches extend 

 throughout the columns; the haemal are complete only in the tail; 

 in the trunk the hajmal spines are absent and the Inemal processes, 

 divided into basal jirooesses and ribs, surround the viscera. A 

 sternum is everywhere lacking. When ossification is lacking or 

 is incomplete, two pairs of arches may occur in each segment, the 

 anterior lieing the stronger and alone persisting in fishes with ossi- 

 fied vertebrte; the second is much smallei-, so that its elements are 

 not called arches, but intercalaria (figs. 55(1, 5S,S). 



The great luimber of visceral arches, and their independence 

 from the cranium, arc; characteristic of fishes. After removal of 

 tliese the cranium in all cartilaginous fishes is very simple (fig. 

 5SS), but in the teleosts, with the appearance of ossification, be- 

 cf)mes very complicated, since the bones are ver}' numerous and 

 are nut, as in mammals, in part fused to larger bones. There arc 

 also I'-reat differences between the different families of fishes, some 

 having l)ones which arc lacking in others (figs. 560, 5S0). The 

 large membrane bones of the cranial roof (parietals, p, frontals, /, 

 and nasals, n(i) iiud the large veittral parasphenoid (px) are 

 (^s|iccial]y constant. The vomer in front (if the ])arasphenoiil is 



