BARNACLES. 275 



Several species of Mediterranean and American crus- 

 tacea may also be seen alive in one or other of these 

 tanks of our public aquaria. 



It is only within a few years that the very large and 

 widely distributed family of marine animals known as 

 " barnacles" ^Cirripedia] have been proved to belong to 

 the same order as crabs and lobsters, and therefore to 

 be veritable crustaceans. Darwin's monograph of these 

 interesting creatures has placed them in a new light. 

 Unlike the evolution of many animals, which begin 

 in a simple way, and gradually pass through em- 

 bryonic stages to a more complex (as the lobster, for 

 example), the barnacles are actually more highly 

 organised when they are young, free-swimming, and 

 crustacean-like, than when they have reached the adult 

 condition. Their life-history is a retrogradation, 

 zoologically speaking, in order the better to adapt 

 them to the very peculiar habits of 



Fig. 206. 



life which we find them affecting. 

 We can group the Cirripedia into 

 two natural divisions, stalked and 

 sessile, of which the stalked barnacle 

 (Lepas anatifera), and the sessile 

 " acorn" barnacle, we find so un- 

 comfortably covering seaside rocks, 

 are relative examples. The beauti- Sessile Acorn Barnacle 



(Balanus porcatus). 



ful plumes or gills protruded from 

 the semi-opened calcareous valves of the former, are 

 well known. These constantly sweep the water in 



T 2 



