STRUCTURE OF THE VERTEBRATES 



91 



swimming instinct well developed, but when speed is required 

 the young are carried on the front flipper of the mother. The 

 teats of some species are partly enclosed by a fold of skin so 

 that the young can be suckled while the mother is swimming 

 slowly. 



The skeleton is highly modified, the digits being enclosed in 

 skin, forming a flipper. The number of phalanges is always in- 

 creased. The pelvic limbs are lost, but a vestigial pelvis is 



Fig. B 



Fig. 39. Order Cetacea. Aquatic, carnivorous placentals including the 

 porpoises and whales. They are the largest known mammals. 



present, and femori have been found in several specimens. The 

 skull is modified, with the nostrils on top of the head, and the 

 frontals reduced. Some of the whales have teeth which are small, 

 very numerous, and similar from front to back; others have 

 only a few, or entirely lack teeth. The last have an epidermal 

 strainer in the mouth, the so-called ''whale bone", which as- 

 sists the animal in catching its food. This consists of enormous 

 numbers of small fish and squid which are taken into the mouth 

 with the water and held by the strainer as the water is ejected. 

 The whales are rapidly disappearing because of modern whal- 



