290 THE WONDER OF LIFE 



to the crab's welfare. It is understood, of course, that as 

 long as the crab is growing, it moults periodically and gets 

 rid of its associates in casting its shell. It is after growth 

 has stopped that the burden tends to become too heavy to 

 be borne. 



The sucking fish Echeneis illustrates the difficulty of 

 classification. It fastens itself temporarily to other fishes, 

 to turtles, and even Cetaceans, but uses them simply as a 

 means of transport. It is no more a parasite than a man 

 on horseback. 



Shelter Associations. No hard and fast lines can be 

 drawn, but it seems useful to group together as ' shelter 

 associations ' a number of interesting cases in which one 

 animal finds shelter in or about another, without itself 

 conferring any benefit in return. 



Fierasfer is one of the best examples of shelter-associa- 

 tion. It goes in and out of sea-cucumbers, starfishes, 

 and big bivalves, but it feeds independently like any other 

 fish. The fact is that it belongs to a family (Ophidiidse) 

 of light-avoiding fishes, such as the sand-eel Ammodytes, 

 and yet is very dependent on the freshness of the water. 

 Thus it occurs in the shelter of animals in which there are 

 active currents of water. 



The entrance of Fierasfer into its sea- cucumber host 

 has been described by Linton. Apparently by accident 

 the fish touches the body of the sea- cucumber (in this ca"se 

 Stichopus moebii), with its snout ; it at once feels its way 

 backward to the posterior end without any pause, as if it 

 was following a scent ; vision does not seem to count for 

 much. When it touches the cloacal opening it brings its 

 slender tail sharply round with a rapid whip-like movement 

 and thrusts the tip in. Up to this point the fish is excited ; 



