78 COMMON BRITISH ANIMALS 



their mouths open, and having entrapped a sufficient 

 quantity of minute crustaceans and pteropods, 

 amounting to several barrels full, they close the jaws. 

 The soft tongue is pressed against the plates of 

 whalebone, which act like sieves, retaining the solid 

 matter and letting the water flow out. The aperture 

 of the throat of all these whales is very small. 



The fin whales, or rorquals, are also whalebone 

 whales, but they are not so profitable when caught 

 and are therefore not right whales. They are dis- 

 tinguished by a small fin on the back and by a 

 smaller head, but the mouth and chest have long 

 parallel furrows. Consequently^ when the mouth 

 opens the gape expands like a purse. 



The giant blue whale [Balsenoi^tera sihbaldi) 

 referred to above belongs to this family of fin whales. 



The toothed whales, which form the second large 

 group of the Cetacea, are more numerous than the 

 whalebone whales and show great diversity of form 

 and size. All the smaller kinds belong to this group. 

 The only toothed whale which approaches the whale- 

 bone whales in size is the cachalot or sperm whale, 

 which attains a length of 50-60 ft. The head of 

 this whale is enormous, being one third the length of 

 the body. It is also characterised by a strange 

 want of symmetry. The blow-hole is single and is 

 situated on the left side of the head. The two nasal 

 passages fuse before they reach the surface, and the 

 left is more highly developed than the right. On the 

 surface of the skull rises a transverse crest of bone, 

 which with the two lateral crests rising from the jaw- 

 bones forms a triangular-shaped basin, wherein lies 



