154 THE STORY OF LIFE IN THE SEAS. 



more of them may be seen hovering round one 

 large Jelly-fish, and we can hardly estimate how 

 valuable to our sea fisheries is the protection 

 afforded by these great Medusae to the young 

 Fish fry. (See Frontispiece.) 



It was not my purpose in writing this book to 

 point out the practical value of scientific investi- 

 gation, but this history of the Jelly-fish and 

 Codling cannot be passed without comment. The 

 Jelly-fish might readily be regarded by the 

 ignorant not only as useless to man, but, in so 

 far as they sometimes choke* his fishing nets and 

 sting his hands and arms, a positive nuisance to 

 him. Scientific investigation when pursued by 

 properly qualified persons for its own sake, and 

 not for any definite commercial results that may 

 possibly come out of it, frequently reveals facts 

 of the utmost importance, such as the one that 

 has just been mentioned. 



There are some other cases of association 

 which would on first consideration be called 

 undoubtedly cases of parasitism, but as this 

 term has been used somewhat vaguely in popular 

 English, it would be well, before proceeding 

 further, to place before the reader a definite 

 statement of the sense in which the word is 

 used in this book. 



In many of the cases that we have mentioned 

 hitherto of animals living together, no apparent 

 injury is inflicted upon either of the associates, 

 but a very definite and decided advantage 

 accrues to each of them, by the association. 



In other cases, however, whilst no apparent 

 injury is inflicted on either, the advantage 



