70. Interestingly enough, several of these hydrodynamic adaptations resemble features designed to improve the aerodynamics of high-speed aircraft.

71. They are also covered with a slick, transparent lid that reduces drag.

72. When not in use, the fins are tucked into special grooves or depressions so that they lie flush with the body

and do not break up its smooth contours.

73. The keels, finlets, and corselet help direct the flow of water over the body surface in such as way as to reduce resistance (see the figure).

74. One potential problem is that opening the mouth to breathe detracts from the streamlining of these fishes and tends to slow them down.

75. Their high, narrow tails with swept-back tips are almost perfectly adapted to provide propulsion with the least possible effort.

76. They can glide past eddies that would slow them down and then gain extra thrust by "pushing off" the eddies.

77. They have evolved special "heaters" of modified muscle tissue that warm the eyes and brain, maintaining peak performance of these critical organs.

78. Although we now tend to refer to the various crafts according to the materials used to construct them-clay, glass, wood, fiber, and metal-it was once common to think of crafts in terms of function, which led to their being known as the "applied arts."

79. The applied arts are thus bound by the laws of physics, which pertain to both the materials used in their making and the substances and things to be contained, supported, and sheltered.