THE JOINTED-FOOT ANIMALS 



167 



sharp-edged shells, composed of carbonate of lime. One 

 species lives on the backs of whales. 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 centi- 

 meters high. At the top of the rock barnacle are two hard, 

 movable valves, meeting in a median line, which, on open- 

 ing, expose long, feather-like processes. These feathery 

 processes are the feet. The animal lies on its dorsal surface 

 within a several-valved shell, 

 and by rapid movement of the 

 feet creates currents of water 

 which bring to the mouth mi- 

 croscopic 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. 

 Though the barnacle in its 

 adult condition, as just de- 

 scribed, has nothing to fear, 

 in early life it swims free at 



the surface of the water in the midst of millions of the 

 young of crabs and other animals. There it is subject to 

 the attacks of animals which might devour it, and many 

 young barnacles are undoubtedly destroyed. At this time 

 it would not be recognized as a barnacle by those who have 

 seen only the adult form. It has an unsegmented body, a 

 long upper lip, single median eye, as in Cyclops, and three 

 pairs of jointed legs. This larval form is called a nauplius 

 and resembles Fig. 93. The barnacle nauplius stage under- 

 goes further complicated changes before it attaches itself 

 by the head to some solid object, as a rock, pile, or ship 

 bottom, when the swimming appendages are absorbed and 

 the shell and feathery foot processes are developed. 



Fig. 93. Nauplius stage of A Hernia. 

 (Much enlarged) 



After Joly 



