692 CARNIVORES xxvu. 7 



The pinnipedes resemble the whales in being microsmatic but have 

 good eyes, with flat cornea, round lens, and a muscular palpebral 

 sphincter. The eyes are directed upwards and prey is often caught 

 from below. The external ears are reduced but hearing is probably 

 acute; the auditory ossicles are massive. There are numerous large 

 vibrissae on the muzzle. The brain is large and rounded, with con- 

 voluted hemispheres and large midbrain and cerebellum. There is an 

 elaborate vocal communication system, the calls varying to human 

 ears from booming to chirping. 



Young seals can remain submerged for up to 25 minutes and have 

 been shown to be able to stand a pressure equivalent to a dive to 95 m. 

 Larger seals can remain submerged even longer and at greater 

 depths. The nostrils are closed by special muscles. The lungs are 

 large and the bronchi contain myoelastic valves. During a dive the 

 heart slows from 120 to 4 beats a minute. There is no drop in blood- 

 pressure, because of a widespread reflex vaso-constriction, which 

 prevents blood reaching the tissues, except the brain and heart 

 muscle. Blood from the brain returns to the abdomen, by a large 

 vessel above the spinal cord, and then accumulates there in extensive 

 sinuses, including a huge dilatation of the vena cava above the liver, 

 which is occluded by a sphincter of striated muscle above the dia- 

 phragm. There are few true retia mirabilia but abundant venous 

 plexuses. The blood can carry as much as 35 c.c. of oxygen for 100 

 c.c. of blood (20 c.c. in man under the same conditions), and there is 

 much myoglobin in the muscles. The respiratory centre tolerates a 

 high C0 2 level. Lactic acid accumulates in the muscles, reducing 

 metabolic levels. By these means the animal is provided with sufficient 

 oxygen for the dive, without absorbing nitrogen and risking 'bends'. 



Copulation takes place in the water in most seals and the penis 

 bone is very large, especially in the walrus. The external genitalia, like 

 the nipples, are withdrawn into folds of the surface. The eggs are 

 fertilized shortly after parturition, the two horns of the uterus carrying 

 alternate pregnancies. Implantation of the blastocyst is delayed for 

 two months or more. As in other carnivores the placenta is zonary, 

 with coloured margins due to the presence of bilirubin. 



In the sea-lions (Otariidae) (Fig. 464) the legs can still be turned 

 forward for use on land, and there are other primitive features, in- 

 cluding external ears. They are more mobile on land than are the seals 

 and can even climb cliffs. The family dates back to the Miocene. The 

 walrus, Odobenus, is a related form, highly specialized for eating 

 bottom-living molluscs, which it digs up with its enormous canines. 



