AKTHEOPODA. 



141 



ing numerous lateral branches, and they are richly supplied with 

 blood. The water which fills the gill-chambers is constantly re- 

 newed by the movements of the legs, and thus the gills are ke])t 

 constantly supplied with fresh water. The nervous system is [jlaced 

 along the ventral surface of the body, and has its usual form. The 

 organs of sense are tlie two pairs of feelers or autennfe, the com- 

 pound eyes, and two organs of hearing. 



B. Anomura. — The most familiar members of this trilie are the 

 Hermit-crabs (Pajtiridw) which occur so commonly on every shore. 



Fig. S'.>.— Brat'liyura. The Spiny Spider-cral) (Maia squinadu). 



They are distinguished by the fact that the abdomen is quite soft, 

 and is not protected by a chitinous crust. The animal, therefoie, is 

 compelled to protect the defenceless part of the body in some arti- 

 ficial manner, and this it effects by a]jpro|jriating the em])ty shell of 

 some dead Mollu.sc, such as the common jieriwinkle or whelk. The 

 abdomen is provided with special apjiendages to enable the intruder 

 to retain firm hold of liis borrowed dwelling, at the same time that 

 he can change it at will when too small or otherwise inconvenient. 



