2688 FISHES 



the mouth, and even on the hyoid bones and gill arches; and they may be attached 

 only to the membrane lining the cavity of the mouth. Frequently they are welded 

 to the underlying bone or cartilage by a broad basis; but, as in the saw fishes, they 

 may be emplanted in distinct sockets. Usually the coating of enamel is very thin; 

 and the ivory, or dentine, is more vascular than in the other classes. In rare instan- 

 ces the ivory may be penetrated by branching prolongations from the central pulp 

 cavity, as well as by similar infoldings from the exterior, thus producing a structure 

 similar to that obtaining in those of the primeval salamanders. As a general rule 

 the teeth are being constantly renewed throughout life, but in a few instances a 

 single set persists. 

 c . In beauty, variety, and changeability, the colors of fishes cannot be 



exceeded by those of any of the other vertebrate classes; metallic tints 

 and almost all the colors of the rainbow being very commonly displayed, while the 

 beauty of the coloration is often enhanced by the rapid changes it undergoes. In 

 many cases the coloration is of a protective nature. An example of this is afforded 

 by the coloring of the upper surface of many flat fishes, such as flounders, which 

 exactly harmonizes with the tints of the sea bottom on which they dwell; while 

 another equally marked instance presents itself in the case of so-called pelagic fishes, 

 like the mackerels and flying fish, which live near the surface of the sea, and have 

 the under parts silvery white, and the back mottled with dark green and black. 

 When viewed from below against the light sky such a fish is practically invisible, 

 while it is equally inconspicuous when seen from above among the dark waters. 

 S ft P ^^ e kdy f fishes is made up of a great lateral muscle on each side, 



divided into a number of segments corresponding with the vertebrae, 

 and also separated into a dorsal and ventral moiety by a median longitudinal groove. 

 On its surface the lateral muscle is marked by a number of white zigzag stripes, 

 generally forming three angles, of which the middle one is directed forward; these 

 stripes being formed by the edges of the tendinous divisions between the segments. 

 Generally the muscles are glistening white in color; but in some instances they are 

 " salmon-colored," this tint being due to the coloring matter of the crustaceans on 

 which such fish subsist, which is turned red by the action of the digestive fluids in 

 the same manner as by boiling. The electric organs possessed by certain fishes are 

 considered to be probably formed from specially developed muscles. With regard 

 to the brain, it will suffice to say that it is of an exceedingly low type; and that the 

 ear lacks the tympanum and tympanic cavity of the higher classes. In the bony 

 fishes there occurs at the base of the brain cavity a sac, often divided into two 

 unequal-sized chambers, and each of which contains an ear bone or otolith of very 

 dense structure. These otoliths, which are very constant in form in the different 

 groups, frequently have scalloped margins and groove-like markings, formed by the 

 ramifications of the auditory nerve. The tongue is frequently absent, and if present 

 is of very simple structure and incapable of protrusion; and since fishes generally 

 bolt their food without mastication, it is probable that they have little, if any, sense 

 of taste. With regard to the digestive organs, it will suffice to mention that in the 

 sharks, rays, and chimseroids the intestine for a large portion of its length is pro- 

 vided with a spiral valve, a similar structure occurring in the ichthyosaurian reptiles. 



