THE JOINTED-FOOT ANIMALS 



151 



crabs and other forms discussed in this chapter, natural- 

 ists have shown that they are in reality closely allied both 

 in development and in structure. The species shown in the 

 accompanying figure is the rock-barnacle (Bal'anus balanoi 1 - 

 des), the most common barnacle along the North Atlantic 

 coast. In some places it literally incrusts the coast-rock 

 between tide lines with its hard, sharp-edged shell, composed 

 of carbonate of lime. The shell is usually a little over a 

 centimeter high, and narrower at the top than at the base. 

 Related species grow to be at least four centimeters high, and 



FIG. 77. Rock-Barnacle. Reduced 



one is found on the back of whales. At the top of the 

 rock-barnacle are two hard, movable valves, meeting in a 

 median line, which, on opening, expose long, feather-like 

 processes. These processes are, morphologically, divisions 

 of the feet. The animal lies on its dorsal surface within its 

 several-valved shell, and by rapid movement of the feet cre- 

 ates currents of water which bring to the mouth microscopic 

 animals and plants as food. When the feet are not scooping 

 in food the valves are closed, forming a most effective armor 

 for the parts beneath. 



