82 



Descriptive Zoology. 



general wavelike motion as if hundreds of hands, with feathery 

 fingers, were constantly opening and closing. Looking closer, he will 

 see that these feathers are in clusters, each projecting from the apex of a 

 cone-shaped body whose base is attached to the rock wall. These are 

 acorn barnacles. Disturb them and they will draw in their feathery 

 appendages and close the shelly valves that guard the opening. They 

 resemble -bivalve mollusks, and in fact were regarded as mollusks until 

 it was discovered that when young they are like the young of the lower 

 Crustacea. After leading a free-swimming life for a time, they attach 



FIG. 54. HERMIT CRAB IN SHELL OF SEA SNAIL. 



themselves by the head end to a rock, and thenceforth live anchored to 

 this one spot. They have given up locomotion, and become sessile. 

 The law of progress in evolution is toward greater freedom, as illus- 

 trated in many forms of animal life ; but here we have a good example 

 of retrograde development, or degeneration. Almost the only ready 

 indication of its crustacean relationship is the segmentation of the 

 appendages. 



Another form of barnacle is the goose barnacle, which has a body 

 resembling a clam, attached by a soft, flexible stalk to some solid 

 object, frequently to a piece of floating timber. When actively feeding, 

 the shell opens and the feather-like feet extend in lively motion, but 



