THE ANIMAL CLIMAX-THE CHORDATES 



347 



they happened to return to an aquatic en- 

 vironment is lost because fossil records are 

 very scanty. They probably were flesh-eat- 

 ing mammals much like the present-day 

 otters, fishers, and minks, which, returning 

 to the water in search of prey, gradually 

 became better and better adapted to ma- 

 rine life. 



Whales have few anatomical features re- 

 maining that are reminiscent of their land 

 life. They have lost their posterior limbs, al- 

 though they still have useless remnants left 

 buried deep in their bodies, and their 

 anterior appendages have modified into ef- 

 ficient flippers. They are lacking hair com- 

 pletely in the adult except around the 

 mouth region in some species. Their nostrils 

 have moved back to the tops of their heads, 

 which facilitates breathing at the water 

 surface without exposing the head. A tre- 

 mendous tafl is the principal organ of loco- 

 motion; it undulates horizontally rather 

 than vertically as in the case of fish. 



Even though the first whales were prob- 

 ably all carnivorous, many subsequent spe- 

 cies became herbivorous. All whales today 

 are divided into two groups : the whalebone 

 whales and the toothed whales. The latter 

 have retained their teeth, which are used 

 in crushing fish and squids, their chief 

 source of food. The more interesting whale- 

 bone whales have evolved a sieve-like struc- 

 ture called whalebone which hangs from 

 the roof of the mouth and is used to strain 

 small marine organisms. Whalebone is ac- 

 tually modified skin, including the hair 

 which makes up the strainer. The sperm 

 whale, also a whalebone whale, has pros- 

 pered on this diet of minute marine life, 

 and some specimens reach a weight of 150 

 tons, which exceeds that of any other ani- 

 mal that has ever lived, dinosaurs not ex- 

 cepted. Oddly enough, they reach this great 

 size by feeding on some of the smallest 

 plants and animals. It has been said that 

 the reason why these animals can grow so 

 fast and so large is that all of the energy 

 received from the food is saved and goes to 



Fig. 13-66. A small beached whole called the beaked 

 whale {Mesoplodon). Note the scratches in the hide 

 as a result of being pounded on the rocks. 



form tissue rather than being lost in keep- 

 ing the animal warm. There is practically 

 no heat loss at all from these great bodies 

 because they are enveloped in a thick layer 

 of blubber just beneath the skin which acts 

 as an insvilator against the cold water in 

 which they live. Another problem that is 

 difficult to understand is how this animal 

 can dive to great depths and remain under 

 water for 30-45 minutes without being 

 crushed and getting the "bends" (see p. 

 488 ) , a disease that humans get under simi- 

 lar circumstances. It apparently has oxygen 

 reserves and other devices for satisfying its 

 needs under these rigorous conditions. The 

 whale is truly a remarkable animal and 

 many aspects of its life will probably re- 

 main a secret for a long time, since it can- 

 not be brought into the laboratory and 

 studied like other animals. 



The primates 



This, the last group of mammals, is the 

 most important of all because we belong 

 to it. Other members of the group are the 

 great apes, the circus monkey, the fierce- 

 looking baboon, the lemurs, and the wide- 

 eyed tarsiers, all coming from an arboreal 

 insectivore ancestor (Fig. 13-67). It is dif- 

 ficult to name any especially striking char- 

 acteristics that set the primates off from 

 all others, though there are several minor 



