THE CRAYFISH AND OTHER CRUSTACEA 



95 



head to some object and gradually assumes the form of a 

 small barnacle. 



The older naturalists classified animals mainly on the 

 basis of external form instead of internal structure. Had 

 they studied the organization of the part of the barnacle 

 within the shell they would have found that the animal 

 really resembles other Crustacea even in its adult state. It 

 has mouth parts consisting of mandibles and maxillae; 

 the feathered appendages which it continually thrusts 

 out and withdraws into its shell while it is in the water 

 are the thoracic legs richly supplied with hairs for entang- 



PIG. 80. A group of barnacles. (After Pilsbry.) 



ling the small creatures used as food. Some barnacles, 

 such as the common goose barnacle, are provided with a 

 flexible stalk, while others such as the acorn barnacles 

 have the shell attached directly to some other object. 

 Barnacles are frequently attached to the hulls of ships 

 where they may be so numerous as to greatly impede the 

 vessel's movements. Some species attach themselves 

 to the skin of whales, others to sea turtles. Some mem- 



