CHAPTER V 

 CLASS II. PISCES (TRUE FISHES) 



The fishes are at present, and have been since Silurian and Devo- 

 nian times, the dominant creatures of the waters, both fresh and salt. 

 The chronological history of the fishes is well shown in Fig. 52. Al- 

 though the environment has been practically constant both as to 

 temperature and chemical constitution the evolution of form and 

 function has gone on rapidly. "This indicates," says Osbom, "that 

 a changing physicochemical environment, although important, is not 

 an essential cause of the evolution of form." 



Although the aquatic environment may in a sense be thought of as 

 constant it is not a uniform or homogeneous medium, for, within 

 aquatic confines, there are several life zones that differ radically 

 from one another. There are: (a) the region of river or tidal currents 

 in which the fish must be active, swift-moving, and predaceous ; (b) the 

 surface strata of still bodies of water, where life may be comparatively 

 passive and where only a moderate speed is necessary; (c) the region 

 at the bottom, which may be either at moderate depths or abysmal, 

 where life may be sluggish. These types and derivatives from them 

 are concisely shown in the accompanying pictorial table (Fig. 53). 



The body form of the type living in currents and depending for 

 food and safety on swiftness is naturally the double-pointed, elon- 

 gated, submarine-shaped animal (Fig. 53, a to c), illustrated by 

 some sharks, the pickerel, the trout, the salmon, and many of the min- 

 nows. Few of these swift-moving fishes have heavy armor, nor have 

 they any excessive development of fins, spines, or other projecting 

 structures that might interfere with swift progress through the 

 water. The dog-shark or common spiny dog-fish (Fig. 62) may be 

 taken as an excellent example of this type of fish, a type that is be- 

 lieved to copy, perhaps more nearly than any other, the ancestral 

 fish form. 



The fishes of the open, seas or lakes living at moderate depths or 

 near the surface, where they are little affected by currents, are often 

 of the deep-bodied, laterally compressed type (Fig. 53 h,i). A good 



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