320 FISHES. 



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 instances 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 through- 

 out life, but in a few instances a single set persists. 



In beauty, variety, and changeability, the colours of fishes 



cannot be exceeded by those of any of the other vertebrate classes ; 

 metallic tints and almost all the colours 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 colouring of the upper surface of many 

 flat-fishes, such as flounders, which exactly harmonises 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. 



The body of 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 

 forwards; these stripes being formed by the edges of the tendinous divisions 

 between the segments. Generally the muscles are glistening white in colour; 

 but in some instances they are " salmon-coloured," this tint being due to the 

 colouring 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 chimaaroids the intestine for a large portion of its length is provided 

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



The gills of sharks, rays, and chimasroids are contained in pouches, usually 



