The Struggle for Existence 149 



densely packed with flinty needles ; another cuts off the tunic of a 

 sea-squirt and throws it over its shoulders ; another trundles about 

 a bivalve shell. The facts recall the familiar case of the hermit- 

 crab, which protects its soft tail by tucking it into the empty 

 shell of a periwinkle or a whelk or some other sea-snail, and that 

 case leads on to the elaboration known as commensalism, where 

 the hermit-crab fixes sea-anemones on the back of its borrowed 

 house. The advantage here is beyond that of masking, for the 

 sea-anemone can sting, which is a useful quality in a partner. 

 That this second advantage may become the main one is evident in 

 several cases where the sea-anemone is borne, just like a weapon, 

 on each of the crustacean's great claws. Moreover, as the term 

 commensalism (eating at the same table) suggests, the partner- 

 ship is mutually beneficial. For the sea-anemone is carried about 

 by the hermit-crab, and it doubtless gets its share of crumbs from 

 its partner's frequent meals. There is a very interesting sidelight 

 on the mutual benefit in the case of a dislodged sea-anemone which 

 sulked for a while and then waited in a state of preparedness until 

 a hermit-crab passed by and touched it. Whereupon the sea- 

 anemone griped and slowly worked itself up on to the back of the 

 shell. 



6 



Other Kinds of Elusiveness 



There are various kinds of disguise which are not readily 

 classified. A troop of cuttlefish swimming in the sea is a beautiful 

 sight. They keep time with one another in their movements and 

 they show the same change of colour almost at the same moment. 

 They are suddenly attacked, however, by a small shark, and then 

 comes a simultaneous discharge of sepia from their inkbags. 

 There are clouds of ink in the clear water, for, as Professor 

 Hickson puts it, the cuttlefishes have thrown dust in the eyes of 

 their enemies. One can see a newborn cuttlefish do this a minute 

 after it escapes from the egg. 



