350 METAZOAN PHYLA 



in swimming to strokes that would serve to direct the fish toward the 

 bottom, and it is, therefore, possessed very largely by those forms which 

 are bottom feeders. The presence of a tail of this type in the shark is 

 correlated with the ventral position of the mouth and the fact that the 

 animal turns over when it seizes an object on the surface. The larger 

 ventral lobe of the flying fish enables it to attain a maximum speed as it 

 leaves the water. A third type is the homocercal type, in which case the 

 two lobes, dorsal and ventral, are about equally well developed. This 

 type of tail is common in the large majority of bony fishes. 



379. Colors of Fishes. — While most of our common fishes do not 

 possess bright colors, some of them are at times very brightly colored. 

 Especially is this true of the males of certain minnows during the breeding 

 season. Still more brilliant, however, are certain fishes in tropical waters, 

 particularly those found on the coral reefs, the colors of which are not 

 exceeded in variety or intensity by any other group of animals. The 

 colors of fishes are due to pigments developed in certain dermal cells known 

 as chromatophores. The main pigments are red, orange, yellow, and 

 black, but various tints are produced by varying combinations of these 

 pigments. These colors are modified by the reflection of light from the 

 scales, which contain crystals of guanin. Blue is such a structure color, 

 and by combinations between it and the pigments a variety of shades 

 of green is produced. The reflection of light from the irregular surface 

 of the scales produces iridescence. White is the result of an absence of 

 pigment. All of these factors together serve to produce the brilliancy 

 and variety of colors which fish present. 



The colors of fish can be modified by the contraction and expansion 

 of the chromatophores, which have an ameboid character (Fig. 251). 

 By means of changes of the chromatophores not only are spots rendered 

 less brilliant or more so but also the general tone of coloration is made 

 to vary considerably. These changes are appropriate to the environment 

 of the fish and may serve to produce a protective coloration. 



380. Internal Anatomy. — The internal skeleton of fishes includes 

 a vertebral column composed of simple vertebrae, the body of each 

 being hourglass-shaped, or amphicoelous. The skull contains a large 

 number of parts and is mostly bony but still contains some cartilage. 

 The pelvic girdle is absent, the ventral fins being attached to a flat 

 bone which is not recognized as representing the pelvis. On the whole 

 the bones which form the skeleton are slender and not very firm. They 

 do not help to support the weight of the body, which is buoyed up by 

 the water, but are only for muscle attachment. 



The digestive system consists of a mouth; a pharynx, the walls of 

 which are pierced by four pairs of gill slits; a short esophagus; a stomach; 

 and an intestine. Teeth are found on the roof of the mouth as well 

 as on both jaws. Three tubular outpocketings, called pyloric caeca, 



