44 ANIMAL LIFE PAST AND PRESENT. 



swim with more or less facility. Our application of 

 the term will, however, in the main be restricted to 

 those -creatures which pass a considerable amount, or 

 the whole of their time in the water, and which have 

 accordingly been more or less specially modified for 

 that kind of life. Again, in many groups of purely 

 aquatic animals, it is sometimes difficult to say which 

 are true swimmers ; a certain number leading an active 

 life when young, and becoming more or less complete 

 fixtures in adult life. We shall commence our survey 

 with the Invertebrate Animals, treating them, however, 

 in a somewhat briefer manner than the Vertebrates, 

 and alluding only to some of the more striking adapta- 

 tions of certain parts of the body for the purpose of 

 swimming. 



All are familiar with those disc-like masses of 

 pellucid gelatinous matter so often thrown up on our 

 sea-beaches, and popularly known as Jelly- Fishes ; but 

 to see them in their full beauty we should look down 

 from the bows of a large vessel traversing the warmer 

 oceans. There they may be seen in countless multi- 

 tudes, extending as far down in the water as the eye 

 can penetrate, and by daylight presenting various tints 

 of pink and purple, while by night they are often 

 phosphorescent. These Medusse, as they are techni- 

 cally called, are gelatinous creatures shaped somewhat 

 like an umbrella, the " handle " being formed by a mass 

 of thick tentacles hanging down in the water. They 

 swim by the alternate contraction and expansion of the 

 " umbrella " or bell ; the diameter of which may con- 



