COMMENSAL1SM AND SYMBIOSIS 173 



91. Commensalism. A curious example of commensalism 

 is afforded by the different species of Eemoras (Echenididce) 

 which attach themselves to sharks, barracudas, and other 

 large fishes by means of a sucking disk on the top of the 

 head (Fig. 103). This disk is made by a modification of 



FIG. 103. Retnora, with dorsal fin modified to be a sucking plate by which the 

 fish attaches itself to a shark. 



the dorsal fin. The Eemora thus attached to a shark may 

 be carried about for weeks, leaving its host only to secure 

 food. This is done by a sudden dash through the water. 

 The Eemora injures the shark in no way save, perhaps, by 

 the slight check its presence gives to the shark's speed in 

 swimming. 



Whales, similarly, often carry barnacles about with 

 them. These barnacles are permanently attached to the 

 skin of the whale just as they would be to a stone or 

 wooden pile. Many small crustaceans, annelids, mollusks, 

 and other invertebrates burrow into the substance of living 

 sponges, not for the purpose of feeding on them, but for 

 shelter. On the other hand, the little boring sponge 

 ( Cliona) burrows in the shells of oysters and other bivalves 

 for protection. These are hardly true cases of even that 

 lesser degree of mutually advantageous association which 

 we are calling commensalism. But some species of sponge 

 " are never found growing except on the backs or legs of 

 certain crabs." In these cases the sponge, with its many 

 plant-like branches, protects the crab by concealing it from 

 its enemies, while the sponge is benefited by being carried 

 about by the crab to new food supplies. Certain sponges 



