OP THE CETACEA. 45 



sketch A, in the ordinary Cetacea, as in the common 

 porpoise. 



As already remarked, the tail of the Cetacea is 

 peculiar ; not vertical, as in fishes, hut horizontal ; 

 hy which great facility is given for their ascent to 

 the surface, to which they must regularly resort for 

 the discharge of the essential act of respiration. 

 The agility of the lesser species, which they owe 

 mainly to the tail, is universally known, and so 

 powerful is it even in the most gigantic varieties, 

 that by its means they frequently force themselves 

 entirely out of the water. This instrument of pro- 

 digious power is formed hy a concentration of the 

 muscles and tendons on all sides of the vertebral 

 column. Mr. Hunter remarks, that the mode in 

 which the tail is constructed is, perhaps, as beauti- 

 ful as to mechanism as any part of the animal; 

 being principally composed of three layers of tendi- 

 nous fibres. It comprises, in the larger species, in a 

 single surface, from eighty to one hundred square 

 feet : its length is only five or six, but its width is 

 from eighteen to twenty-six. In its form it is flat 

 and semilunar ; its motions are rapid and universal ; 

 its strength immense. It is nearly the sole instru- 

 ment of defence as well as of motion. The greatest 

 velocity is produced by powerful strokes against 

 the water impressed alternately upwards and down- 

 wards, but a slower motion is produced by cutting 

 the water laterally and obliquely downwards, in 

 a similar manner as a boat is forced along by a 

 single oar in the operation of skulling. So rapid 



