Crustaceans. 



49 



[Ligia occanica) is one of the largest British Isopods, found 

 running on rocks (Fig. 56). Although essentially terrestrial, the 

 sea-slater never occurs far from the sea, thus differing from its 

 near relatives, the wood-lice and pill-bugs. Most Isopoda 

 are aquatic, and many species of small size occur among weeds 

 in rock-pools. The little Enridicc pulchra, only two lines long, 

 is a thoroughly aquatic Isopod, found swimming in piuddles on 

 the sandx' beach, as well as in the sea, where it is said to bite 

 people bathing. 



The Crustaceans dealt with so far are free-living and exhibit 

 the full characteristics of the group. But a large number of 



56- 



-SEA-SLATLr 



(AFTER BARS 



forms, belonging to nearly all the principal divisions, hax-c liecome 

 adapted to a very different mode of existence, have lost the 

 power of locomotion, and live attached to stones, boats or larger 

 animals. Such fixed Crustaceans are for the most pail curiously 

 degenerate, and their position in tlie zoological s^'stem could 

 onl}' be ascertained by a knowledge of their development. 

 The most familiar of these forms are undoubted!}' the Cirripedes 

 or Barnacles. We all know the acorn barnacles, or acorn-shells 

 (Balanus). which cover rocks and stonework washed by the tides, 

 to the inconvenience of bathers and bare-footed investigators of 

 the rock-pools (Fig. 54). When seen attached to a rock at low 

 tide, a barnacle shows no signs of life, the body being enclosed in a 



