10 



A HISTORY OF FISHES 



Bonito {Gymnosarda) (Fig. 4) is one that is admirably adapted 

 from a mechanical point of view for cleaving the water, and is 

 that which is clearly best suited for progression in that medium. 

 The mechanical conditions which led man to construct his 

 submarine to a certain pattern, in order to have a vessel that 

 would move freely in all directions under water, are precisely 

 the same as those which have determined the shape of the 

 fish's body, so that it is not surprising to find that the form of 

 the animate fish corresponds closely with that of the man-made 

 submarine. 



The shape of the body of a Mackerel is fusiform ; that is, it is 

 shaped somewhat like a cigar, circular or elliptical in cross- 



Fig. 4. — A SWIFT PELAGIC FISH. 

 Oceanic Bonito {Gymnosarda pelamis), X g. 



section and thicker in front than behind. It has been described 

 by one authority as resembling a double wedge, the thick part 

 of which is represented* by the head and anterior part of the 

 body, and one of the thin edges by the free hinder margin of 

 the tail. Every line of its smooth, rounded contour is sug- 

 gestive of swift motion, there being an almost complete absence 

 of irregularities or projections calculated to hinder progression. 

 There is no distinct neck as in the land vertebrates, the head 

 merging insensibly into the trunk and the trunk into the tail, 

 the boundaries between these regions of the body being denoted 

 by the gill-opening and vent respectively. Viewed from the 

 front, the outline of the fish appears as a perfect ellipse of 

 comparatively small size (Fig. 5A). The beautifully moulded, 

 bullet-shaped head, with its pointed snout forming an efficient 

 cutwater; the jaws fitting so close together that it is scarcely 

 possible to insert the blade of a penknife between them; the 



