252 



ANALYSIS OF THE ENVIRONMENT 



defense, while giving benefit to their sessile 

 partners by their locomotion and consequent 

 avoidance of stagnation of the water of the 

 immediate environment, and by their trans- 

 fer from feeding area to feeding area. The 

 relation of decapod crustacean and sea 

 anemone is facultative in so far as the juve- 

 nile animals are concerned, but older indi- 

 viduals seem to be always in partnership, 

 and the obligate nature of the relations 

 is then evident in the evolution of distinct 



species of hermit crabs. These crabs begin 

 their snail-shell-inhabiting career without 

 the Sagartia, and the juvenile Sagartia may 

 be found on stones, unassociated with the 

 crabs. 



The diflBculty presented to the individual 

 crab, when it must exchange its snail shell 

 house for a larger one, of preserving its pro- 

 tective attendant, and equally the dangei 

 faced by the sea anemone of being left be- 

 hind on the old shell are met in other mu- 



Fig. 68. The mutualist sea anemone, Adamsia palliata, associated with the hermit crab, 

 Eupagurus prideauxi. A, The hermit crab, its snail-shell house almost concealed by two sea 

 anemones. B, The shell and sea anemones abandoned by the crab. C, The empty shell, show- 

 ing the extension of the opening produced by secretion from the foot of the sea anemone. 

 ( From Hesse and Doflein, ) 



species of the attached forms limited to this 

 habit and to a particular species of crab. 

 Each becomes a dominant biotic environ- 

 mental factor in the hfe of the other. 



The intimacy of this completely external 

 type of mutualism is reenforced by the 

 development of special structures by the 

 sessile partner. In a simple type of sea 

 anemone-hermit crab relation, the crab has 

 been reported to transplant the sessile as- 

 sociates to a new shell when it has out- 

 grown the old one and is forced to change. 

 A single crab may bear several sea anem- 

 ones. The sea anemone Sagartia parasitica 

 is reported from several North Atlantic 



tuaUsts of the hermit crab-sea anemone 

 series by the modification of the foot of the 

 sea anemone to foiTn an extension of the 

 snail shell house. This reaches an extreme 

 in the relation of Eupagurus prideauxi with 

 the sea anemone Adamsia pileata (Fig. 68). 

 Still other crabs bear small sea anemones on 

 one of their claws, or hold one in each 

 claw. The effective defense provided by the 

 sea anemone against so formidable a pred- 

 ator of crabs as an octopus is reported 

 from observations made at the Naples 

 Aquarium. The mutualistic actinians have, 

 in fact, extremely well-developed nettle 

 cells, and usually belong to genera in which 



