INTRODUCTION xvii 



without exception, on the highest point of the head and usually 

 at some distance from the tip of the snout (Fig. 2). The eye 

 of a fish is built on the same general plan as that of other 

 Vertebrates, but is somewhat modified for vision under water. 

 Neither fishes nor cetaceans show any traces of external ears. 

 In a fish there is no visible external aperture, and the internal 

 organ associated with the sense of hearing is of comparatively 

 simple design. In a cetacean the aperture is situated between 

 eye and edge of flipper, and is so small and inconspicuous that 

 even in a 100-foot monster its diameter is not greater than 

 that of a lead pencil. 



The mouth, both in fishes and cetaceans, varies greatly, not 

 only in size and shape, but also in position. In fishes the 

 jaws are usually provided with teeth of one form or another. 

 Among the cetaceans, teeth are present only in the members 

 of the sub-order Odontoceti, the Toothed Whales (p. 256), and, 

 although they are very variable in number and size, they do 

 not show any differentiation into incisors, canines and molars 

 in any living species. They are all uniform, usually conical in 

 shape, and the range in number extends from 1 to more than 

 50 in a tooth row. Teeth may be present in both jaws, or 

 they may be restricted to either the upper or the lower jaw. 

 In certain forms such teeth as persist are concealed beneath 

 the gums throughout life, so that the animals appear to be 

 toothless. The members of the sub-order Mystacoceti, the 

 Whalebone Whales, feed in quite a different manner, and 

 instead of teeth, the characteristic whalebone or baleen is 

 developed. This is horny in structure, and is suspended in two 

 rows of flattened plates from the under surface of the rostrum 

 (p. 203). The two skulls illustrated here (Figs. 4, 5) show the 

 main differences between the mouths of the Whalebone and 

 Toothed Whales. 



With the organs of respiration we encounter one of the most 

 important differences between the fishes and cetaceans. The 

 fish obtains the oxygen necessary for life from the air dissolved 

 in the water by means of special organs known as gills. When 

 a fish breathes, water is taken in through the mouth, and, after 

 passing over the giil-plates or gill-filaments, is expelled through 

 the external gill- openings in the sides of the " neck ". These 

 plates are richly supplied with fine blood-vessels, in which the 



