ICHTHYOPSIDA. 



CHAPTER XXIV. 

 CLASS I. PISCES. 



THE fishes form the lowest class of the Vertebrata, and 

 they may be broadly defined as being Vertebrate animals pro- 

 vided with gills, whereby they are enabled to breathe air dis- 

 solved in water ; the heart, when present, consists of a single 

 auricle and ventricle (with the exception of the mud-fishes) ; 

 and the limbs, when present, are in the form of fins, or expan- 

 sions of the integument. 



In their external form, fishes are in most cases adapted for 

 rapid locomotion in water, the shape of the body being such 

 as to cause the least possible friction in swimming. To this 

 end, as well as for purposes of defence, the body is generally 

 enveloped in a species of chain-mail formed by overlapping 

 scales, to which bony plates, tubercles, and spines, are some- 

 times added. Valuable characters can sometimes be drawn 

 from the nature of the scales, and with a view to this the 

 integumentary appendages of fishes have been divided by 

 Agassiz as follows (Fig. 100) : 



1. Cycloid scales (a), consisting of thin, flexible, horny 

 scales, which are circular or elliptical in shape, and have a 

 smooth outline. These scales occur in most of our common 

 fishes (e. g., the pike). 



2. Ctenoid scales (b). These resemble the cycloid scales 

 in being thin, flexible, horny scales, but they are distinguished 

 by having their hinder margins cut into comb-like projections, 

 or fringed with spines. The common perch supplies a good 

 example of these scales. 



3. Placoid scales (c). These are detached bony grains, 



