92 WHALES 



the animal's condition. Lice are not the causes of disease, they simply 

 multiply far more readily in sick than in healthy specimens. In whales, a 

 contributory factor may well prove to be their seasonal habitat. This is 

 certainly true for another group of external parasites: diatoms (Fig. 133). 

 These microscopic plants often form a yellowish film over the sides and 

 bellies of some whales and dolphins, and particularly of Antarctic Blue 

 and Fin Whales. When such animals are hauled up on to the deck of a 

 factory ship, they often look like an entirely different species, whence the 

 name "sulphur bottom' for some Blue Whales. Once this filmy layer is 

 scraped off, however, the normal colour reappears. 'Sulphur bottoms' are 

 most prevalent in the Antarctic, simply because diatoms are most wide- 

 spread in this region (cf. Chapter 12). 



Our long discourse on the whale's external appearance has led us from 

 our giant's early beginnings to the lice which infest its gigantic bulk. We 

 have seen that the external features of this strange animal still keep a great 

 number of secrets from us, and when we come to discuss its internal organs 

 and its behaviour in subsequent chapters, we shall see that these secrets 

 are bv no means its onlv ones. 



