288 THE WONDER OF LIFE 



FIG. 47. Epizoic growth of hydroid polyps Hydractinia (A), on shell of 

 whelk Buccinum (B), which is tenanted by a hermit-crab. For- 

 ceps of hermit-crab (c). The hydroid colony shows division of 

 labour; it includes nutritive, reproductive, and other types of 

 individual. 



epizoic animals : acorn-shells on bivalves and crabs, 

 Serpulid worms on shells, Hydrozoa and Polyzoa on many 

 kinds of marine animals, one sponge on another and so on. 

 Weber refers to the fact that the muddy floor of the Banda 

 Sea is covered for miles with a dense network of a large 

 Foraminifer, Rhizammina algceformis, and that this serves 

 as a suitable substratum for a large number of sedentary 

 animals, which could not otherwise find a foothold in the 

 soft mud. 



Of many of these epizoic marine animals it must simply 

 be said that they grow upon other marine animals just as 

 they might grow on any other object. The young stages 

 happened to land there and found the substratum suitable. 

 This must be true of the acorn-shells (Balanns), false-oysters 

 (Anomia), serpulid worms, Polyzoa, zoophytes, and the 

 like often found on crabs, which do not seem to illus- 

 trate more than fortuitous epizoic association. But some 



