xxvi FEEDING 669 



flesh of large whales. Squids are also eaten, but in species that feed 

 predominantly on these there is some tendency to a reduction in the 

 number of teeth. For example, in the sperm whale functional teeth 

 are confined to the lower jaw and in the bottle-nosed or beaked whales 

 only one or two pairs of teeth are visible. In whalebone whales 

 (Mysticeti), however, there are teeth only in the foetus; the food is 

 plankton. This is collected by the fringed baleen, which consists of 

 rows of transverse plates of keratin. The tongue of the right whales is 

 powerfully muscular and forces the water from the mouth. In the 



Fig. 437. Skeleton and outline of the right whale {Balaena). 

 (After British Museum Guide.) 



rorquals the same purpose is achieved by the contraction of sub- 

 cutaneous muscles associated with the external throat grooving. The 

 shrimp-like 'krill' {Euphausia) and other organisms are then swallowed 

 through the narrow oesophagus to a special stomach with several 

 chambers. The first stomach is non-glandular and is a crop-like 

 oesophageal specialization, the hindmost stomach into which the bile 

 and pancreatic ducts open is a specialized intestinal cavity. The in- 

 testine may be as much as sixteen times as long as the body. 



Both types of nutrition are evidently efficient and the whales are 

 abundant and of course very large. It is more easy to see the advantage 

 of large size for aquatic than for land animals. There are no problems 

 of support of weight, and on the other hand a great premium is placed 

 on large size by the fact that skin friction is thereby relatively reduced, 

 and this, which forms but a small element in the work to be done by 

 a land animal, must be important in the water. Further, the heat loss 

 is greatly reduced by the size, and this may be a large factor in cold 

 water, with its high thermal conductivity (Parry, 1949). However, it 

 has also been claimed that downward temperature regulation may be 

 a problem and that the flippers and fins act as 'radiators'. 



The respiratory system shows many special developments in the air 

 passages, lungs, and nostrils. There are valves for closing the nostrils 



