CHAPTER IV 

 FISHES 



The Class Pisces, or fishes, is the largest class in number of 

 genera. With a few exceptions they are completely aquatic in 

 habit, and are distributed throughout the world in both fresh 

 and salt water. In their anatomical characteristics they show 

 a wide variation, as would be expected from their wide geograph- 

 ical distribution, and the enormous length of their racial his- 

 tory. The group may be defined as aquatic vertebrates with 

 fins, gills, and dermal scales. The fish evolved from a cyclostome 

 type of ancestor at such an early period in vertebrate history 

 that they may almost be considered as parallel lines of develop- 

 ment. The following list of characters will help to show their 

 advance over the cyclostomes, and give an idea of their funda- 

 mental structure. 



1. Fins. The fish have two pairs of appendages, pectoral and 

 pelvic, in addition to the dorsal, ventral and caudal fins. The 

 fins are characteristic in having the distal (outer) border of 

 dermal rays, entirely unlike the feet and toes of more advanced 

 vertebrates. In several highly specialized recent fishes the fins 

 have been lost. Such cases are degenerative specializations. 



2. Jaws. Like all higher vertebrates, the fish have developed 

 jaws for holding, biting, or crushing food. The jaws are modified 

 gill arches. 



3. Scales. Dermal scales are found in the fish and set them 

 apart from the cyclostomes. Such scales arise in the mesodermal 

 tissues of the skin (the dermis) and are developmentally dif- 

 ferent from epidermal structures. Fish scales vary from minute 

 bony dermal denticles in the most primitive group of fish to 

 heavy bony plates, or flexible dermal scales in the typical food 

 fish. In a number of specialized forms the scales are almost, or 



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