2: .STWGTURA-L AND SYSTEMATIC ZOOLOGY 



mented, and fixed or parasitic in adult life, growing 

 head downward in their shell. The feathery, branched, 



FlG. 57. Barnacles, or Pedunculate Cirripedes (Lepas anatijera). 



thoracic feet are protruded through the opening of the 

 shell to grasp particles of food. Lepas, the ship 

 barnacle, grows attached to floating timber and the 

 bottoms of ships by a long, leathery 

 stalk (Fig. 57). The acorn shells 

 (Balamis} grow on rocks between 

 tide marks, their white, conical 

 " shells " forming an incrusting 

 layer on the rock (Fig. 58). 



5. Decapoda ; large, highly or- 

 ganized crustaceans, having usually 

 a thorax of eight and an abdomen 

 of seven segments, the anterior re- 

 gions of the carapace united to form a cephalothorax ; 

 the eyes are borne on stalks and the gills are thoracic. 



FIG. 58. Acorn Shells (Ba- 

 lanus} on the shell of a whelk 

 (Bucctnum), 



