ICHTHYOPSIDA. 227 
iCcCHTAY OPSTa. 
CHAPTER XXIV. 
CLASS I. PISCES. 
THE fishes form the lowest class of the Vertebrata, and they 
may be broadly defined as being Vertebrate animals Jrovided 
with gills, whereby they are enabled to breathe air dissolved 
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 zz the form of fins, or expansions 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 en- 
veloped in a species of chain-mail formed by overlapping scales, 
to which bony plates, tubercles, and spines are sometimes 
' 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. 122) = 
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 (¢.¢., 
the pike). 
2. Ctenoid scales (0). 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, 
