COMMENSALISM AND SYMBIOSIS 173 
91. Commensalism.—A curious example of commensalism 
is afforded by the different species of Remoras (Zchenidid@) 
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.—Remora, with dorsal fin modified to be a sucking plate by which the 
fish attaches itself to a shark. 
the dorsal fin. The Remora 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 Remora 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 ° 
