CRUSTACEA. 



135 



In the olden days before the introduction of copper-sheathing, the hulls of 

 ships formed a favorite support for these animals, and there they grew in 

 such numbers and to such a size that they seriously impeded the progress 

 of the vessel through the water. Hence at intervals the bottom of the ship 

 had to be scraped and cleaned. The numbers of them which would accu- 

 mulate after a voyage in tropical seas is almost beyond belief. At one time 

 about three hundred tons of them were removed from the hull of the ' Grc.it 

 Eastern.' This habit of becoming attached to floating objects has had con- 

 siderable influence on their geographical distribution. While most animals 



Fig. 122. — Goose-barnacles (Lepas), attached to a piece of floating pumice-stone. 



occur only in certain well-defined limits, the species of barnacles are almost 

 cosmopolitan. The common forms of the Pacific occur in the Atlantic 

 as well; temperature being about the only external condition affecting 

 them. 



The ten-footed Crustacea (Decapods) are but remotely related to the 

 barnacles, and their present juxtaposition is due to the fact that we fol- 

 lowed out first one branch of the crustacean tree, and now have to follow 

 out another. The decapods are connected with the phyllopods by means of 



