176 INTRODUCTION TO ZOOLOGY CHAP. 



of six pairs of two -branched thoracic legs, which project 

 beyond the shell when it is open and drive the food into 

 the mouth. The abdomen is reduced to a little stump. 



Most barnacles are hermaphrodite, and the long tube (penis) 

 from the spermary may sometimes be seen projecting from 

 the shell beyond the thoracic limbs (Fig. 111). The sense 

 organs are all degenerate, except for the tactile hairs present 

 on the limbs. 



The barnacle is a striking example of modification of 

 structure resulting from a change of habit. 



The Stalked Barnacles, or Goose Barnacles (Lepas anatifera), 

 are very similar in structure to the Acorn Barnacles except 

 for the presence of the long fleshy stalk formed from the 

 region of the head below the first antennae, this region 

 having become much swollen and elongated. 



These barnacles often become so numerous at the bottom 

 of a ship that they have to be scraped off by divers when 

 the ship is in harbour. 



The name "Goose Barnacle" refers to the old legend, 

 apparently of Scotch or Irish origin, that from each barnacle 

 shell there hatched out a complete little bird like a 

 miniature goose. This legend was believed even in the 

 seventeenth century, when it was inscribed in the Trans- 

 actions of the Royal Society \ 



General Classification of the Crustacea mentioned in Chapter XII. 



I. Higher Crustacea (Malacostraca). Those with a definite 

 number of segments and appendages, and with a 

 carapace formed from the dorsal exo-skeleton of the 

 fused segments of the head and the whole of the 

 thorax. 



Order 1. Decapoda. Those with five pairs of walking 



legs and with compound eyes. 

 (a) Long-tailed forms : Prawn, Lobster, etc. 

 (6) Short-tailed forms : Crabs. 

 (c) Soft-tailed forms : Hermit Crabs. 



Order 2. Amphipoda. In this order the carapace only in- 

 cludes one, or a few, of the thoracic segments, leaving 

 others free ; the body is laterally compressed ; the 

 gills are attached to the thoracic legs, which are 



