THE ARTHROPODS 



3T5 



FIG. 126. A goose- 

 barnacle, x, piece 

 of wood to which 

 the animal is at- 

 tached ; n, neck ; 

 mouth is opposite 

 m; s, bivalved 

 shell ; /, feet which 

 move towards the 

 mouth (m). (From 

 Smith.) 



clams, and are still so placed in the popular 



classification of many fishermen. About 



1830 a study of the developing barnacle 



eggs showed that each egg forms a small 



triangular larva (nauplius) with three pairs 



of legs (Fig. 127). In this condition very 



many crustaceans hatch. This barnacle 



larva swims for a time and finally settles 



down on a floating object (or perhaps a 



rock in case of the rock-barnacles), and 



metamorphoses into a barnacle. The larvae 



literally swarm in tropical waters, and pass- 

 ing ships get their hulls coated with myriads 



of barnacles. These may grow to be sev- 

 eral inches long during a voyage of a few 



months ; and such a growth, of course, greatly impedes the 



vessel. 



A strange thing about the barnacle is that the point of 

 attachment to rocks and timbers is the 

 head ; and this led Huxley to define a 

 barnacle as " a crustacean fixed by its 

 head and kicking its food into its 

 mouth with its legs." This is literally 

 true, for the long legs projecting from 

 between the valves of the shell do drive 

 food towards the mouth. 



Certain barnacles become attached 

 to crabs, and develop root-like pro- 

 cesses which penetrate the tissues of 

 the crab and absorb nutriment. Then 

 the barnacle degenerates until it is 

 !Te C gTnning nothing but a sac-like tumor on the 

 of the ovary ;' i,2,s t the body of the crab, and the sac contains 



three pairs of legs present j th reproductive Organs of the 

 at hatchi-ng. (From J 



barnacle. Such extreme degeneration 



FIG. 127. The nauplius 

 stage of a barnacle, oc, 

 eye-spot ; ov 



