ORGANIC PARTNEESHIPS OR SYMBIOSIS 



163 



hermit-crab and attempt to draw it out of its shell with the point of 

 one of its eight arms. But before this had succeeded there sprang 

 from the body of the sea-anemone a large number of thin worm-like 

 threads which spread over the arm of the robber, who immediately let 

 go his hold of the crustacean and troubled himself no further with it. 

 These threads, called acontia, are thickly beset with stinging-cells, 

 which must at least cause a violent smarting on the soft skin of the 

 octopus. Thus we see that the Actinia instinctively defends its 

 partner from attacks, and does it so effectively that we need not 

 wonder how the instinct to provide itself with Actiniae could have 

 arisen in the hermit-crab. But the acontia seem to have been greatly 



Fig. 34. Hermit-crab {E), -within a Gasteropod shell, on wliich a colony 

 of Podocorijne carnea has established itself. From the common root-work (whic-h 

 is not clearly shown) there arise numerous nutritive polyps with tentacles {np), 

 among which are smaller ' blastostyle ' pol3'23s with a circle of medusoid buds 

 (mk), spine-like personae {sfjy), and on the margin of the mollusc shell a row 

 of defensive individuals {wp). F, antennae. Au, eyes of the hermit-crab ; 

 slightly enlarged. 



strengthened in the course of the sea-anemone's association with 

 hermit-crabs, for they do not occur in all forms, and they are most 

 highly developed in those which live in Symbiosis with crustaceans. 



In this case the structural change, the transformation of the 

 mesenteric filaments that occur in all Actinias into projectile acontia, 

 is comparatively slight, but in another partnership between hermit- 

 crabs and polyps the latter have undergone a much more marked 

 adaptation. At Naples Etvpagurus j^rideauxii is one of the com- 

 monest hermit-crabs. It lives at a depth of about a hundred feet, 

 and is often brought to the Zoological Station by the fishermen in 

 large quantities. Its borrowed mollusc shell often bears a little 

 polyp, Podocoryne carnea (Fig. 34), which forms colonies of often 



