BARNACLES AND OTHER CRUSTACEANS 113 



wall or cone. The creature is thus actually fastened by 

 its head — " upside down, with its legs sticking up " not 

 in the air, but in the water. Those six pairs of Y-shaped 

 legs, though no longer enabling the barnacle to swim, 

 increase in relative size, and keep up their active move- 

 ments. It is they which emerge like a plume when the 

 valves of the shell open and carry on the rhythmic 

 bowing and scraping movement described above. 



The barnacles have, in fact, undergone a transforma- 

 tion which may be compared to that experienced by a 

 man who should begin life as an active boy running 

 about as others do, but be compelled suddenly by some 

 strange spell or Arabian djin to become glued by the 

 top of his head to the pavement, and to spend his time 

 in kicking his food into his mouth with his legs. Such 

 is the fate of the barnacles, and it is as strange and 

 exceptional amongst crustaceans as it would be amongst 

 men. Indeed, to " earn a living " human acrobats will 

 submit to something very much like it. It is this change 

 from the life of a free-living shrimp to that of a living 

 lump, adherent by its head to rocks or floating logs, 

 that Vaughan Thompson in 1830 discovered to be the 

 story of every barnacle, and so showed that they were 

 really good crustaceans gone wrong, and not molluscs. 

 It is a curious fact that the young ascidian or sea-squirt 

 which swims freely and has the shape of a tadpole, also 

 when very young fixes itself by the top of its head to a 

 rock or piece of seaweed, and remains immovable for the 

 rest of its life. Though agreeing in their strange fixation 

 by the head, the barnacle and the ascidian are very different 

 kinds of animals. (For some account of the Ascidian the 

 reader may consult the chapter " Tadpoles of the Sea " in 

 " Science from an Easy Chair," Second Series. Methuen, 

 1912.) 

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