498 VERTEBRATE LIFE AND ORGANIZATION 



Desjjite these fish-like attributes, cetaceans are air-breathing, viviparous 

 and suckle their young (Fig. 24.17). 



Most cetaceans have a good complement of conical teeth well suited 

 for feetling upon fish, but the largest whales have lost their teeth and 

 feed upon plankton. With fringed, horny plates (the whalebone) that 

 hang down from the palate, a toothless whale strains these minute organ- 

 isms from water passing through its mouth. The richness of the plankton 

 together with the buoyancy of the water has enabled these whales to 

 attain enormous size. The blue whale, which reaches a length of 100 

 feet and a weight of 150 tons, is the largest animal that has ever existed. 



Ungulates. Horses, cows and similar mammals have become spe- 

 cialized lor a plant diet. This has entailed a considerable change in their 

 dentition, for plant food must be thoroughly ground by the teeth before 

 it can be acted upon by the digestive enzymes. The molars of plant- 

 eating mammals (and those of omnivorous species sucli as man) have 

 become square, as seen in a surface view. Those of the upper and lower 

 jaws no longer slide vertically across each other to give some cutting 

 action, as do the triangular molars of more primitive mammals, but meet 

 and crush the food between them (Fig. 24.12). A simple squaring of the 

 molars, and to some extent of the premolars, is sufficient for herbivorous 

 mammals that browse upon soft vegetation. But those that feed upon 

 grass and other hard and gritty fare, as do the grazing species, are con- 

 fronted with the additional problem of the wearing away of the teeth. 

 Two adaptations have occurred: the height of the cusps of the teeth has 

 increased, and cement (a hard material previously found only on the 

 roots of the teeth) has grown up over the surface of the tooth and into 

 the "valleys" between the elongated cusps. More tooth is provided to 

 wear away, and the tooth is more resistant to wear. Teeth of this type 

 are referred to as high-crowned in contrast to the more primitive low- 

 crowned type. 



Herbivores constitute the primary food supply of carnivores, and 

 protect themselves primarily by the simple expedient of running away. 

 Speed has been increased by the evolution of an unguligrade foot pos- 

 ture, i.e., lengthening the foot and standing on the toe tips (Fig. 24.18). 

 Those toes that no longer reach the ground became vestigial, or disap- 

 peared, and the primitive claw on the remaining ones was transformed 

 into a hoof— a characteristic that gives the name ungulate to these 

 mammals. 



The numerous and varied contemporary ungulates are grouped into 

 two orders that can be separated on the basis of the type of toe reduc- 

 tion. In the order Perissodactyla, the axis of the foot passes through the 

 third toe, and this is always the largest. Ancestral perissodactyls, includ- 

 ing the primitive forest-dwelling horses of the early Tertiary, had three 

 well developed toes (the second, third and fourth) and sometimes a trace 

 of a fourth toe (the fifth). The tapir and rhinoceros, which still walk 

 upon soft ground, retain the middle three toes as functional toes, but 

 only the third is left in modern, plains-dwelling horses. Perissodactyls 

 are characterized by having an odd number of toes. 



In the order Artiodactyla, the axis of the foot passes between the 

 third and fourth toes, which are equal in size and importance. Ancestral 



