PARASITES AND MESSMATES 209 



from the Turtle itself; neither is it a commensal, 

 for there is no evidence that it shares in the Turtle's 

 meals. It probably takes to a Turtle, when it can 

 find one, as giving it a wider range of operations than 

 is afforded by its usual drift-log or tuft of sargasso- 

 weed. A somewhat similar case is afforded by some 

 of the Barnacles that are found on the skin of 

 Whales. The species of Conchoderma, for instance, 

 are often found on certain Whales, but they may 

 also occur on inanimate floating objects. Other 

 Whale-infesting Cirripedes, however, are specially 

 adapted to their habitat, and never occur elsewhere. 

 For example, Coronula (Plate XXVIII.) is a genus 

 of sessile Barnacles in which the shell is elaborately 

 folded, forming a series of chambers into which pro- 

 longations of the Whale's epidermis grow, securely 

 fixing the shell. Tubicinella is even more effectively 

 protected against dislodgment, for its shell is sunk 

 in the thickness of the Whale's skin, with only the 

 opening exposed. Other genera of sessile Barnacles 

 (Chelonobia, etc.) are found adhering to the shell of 

 Turtles. The increased food-supply made available 

 by the host's movements through the water is 

 probably the chief advantage that the Barnacles 

 gain in such cases. This is indicated by the fact 

 that certain small stalked Barnacles (Dichelaspis, etc.), 

 found on large Crabs and Lobsters in tropical seas, 

 generally cluster on the mouth parts of their hosts, 

 near the entrances to, or even within, the gill 

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