182 LECTUEES ON BIOLOGY 



by eating it up. Only the head with the powerful claws protrude 

 from the shell, the rest of the body is concealed inside. As 

 result of this mode of life the abdomen of the hermit-crab has los 

 its natural armour which has been rendered unnecessary by th 

 shell of the snail, and has become soft, offering a favourable poir 

 of attack to the crab's numerous enemies. But the animal is we 

 aware of its weak side, and does not abandon its artificial armon 

 except for the most urgent reasons. Only when it has outgrow 

 the dimensions of its house it leaves it, to exchange it mime 

 diately for a bigger domicile. Thus the hermit-crab, clad i 

 armour, moves along on the ocean-bed in search of its prey. 



Attached to the shell are always found one or more sea 

 anemones (Adamsia rondeletii], which are thus carried throug 

 life by the hermit-crab. It is not difficult to see the advantag 

 which the Actiniae derive from this remarkable arrangemem 

 They use the legs of the crab as a means of locomotion for thei 

 own voyages in search of prey, and take at the same time 

 moderate toll of the repasts of their hosts. It is more dimcul 

 to understand the advantage which the sea-anemones grant t 

 their host in return for these services, for unless the hermit 

 crab derived some advantage for itself from this alliance it woul 

 probably leave its house with the anemones on it and see' 

 another domicile. But I was soon to be shown this side of th 

 1 contract.' 



One day, by the mistake of an attendant, a large octopu 

 (Octopus vulgaris), one of the most dangerous enemies of th 

 hermit-crab, had been put into the tank, and at once commence" 

 to attack the crab, seizing it with its strong tentacles covere< 

 with powerful suckers, and endeavouring to pull it out of th 

 snail-shell. Resistance seemed quite hopeless, but the octopu 

 had reckoned without the ' sea-anemone,' for no sooner had i 

 seen the danger which threatened its host than it threw it 

 numerous poison-lassoes on the naked arms of the attacker 

 The octopus at once let go its prey and took to flight with ever 

 sign of pain and terror. 



How well the hermit-crab understands the security grante< 

 by the presence of its friend is easily demonstrated. If w< 

 forcibly remove the crab from its house, a process during whicl 

 we are exposed to a painful cannonade from the batteries of th< 



