112 



ANIMAL LIFE 



surface of the head, a rapid movement may be observed 

 at the sides of the mouth, indicating the position of the 

 bailer ; the gills are, in fact, enclosed in such a way that 

 the spate around them is converted into a tube, open 

 in front and behind. 



In a burrowing animal, however, such a method 

 entails the danger of sucking up debris and choking 



Fig. 25. — Head and thorax of Crayfish, to show the gills and the bailer, ep y 

 {From Lang's ' Comparative Anatomy.') 



a it itj, antenna; ; at\, at>., abdominal segments ; ptb t3 , pdh,,, frdb^, gills ; //,, first 

 swimmeret ; 5-13, thoracic appendages. 



the gills. Accordingly we find among burrowing 

 crabs a great variety of devices which, while per- 

 mitting them to escape their numerous foes — the 

 larger fish — allows them to maintain an in- and out- 

 going current without blocking the gills. The common 

 crab, which sinks up to the eyes in soft sand at the 

 slightest warning, has a reversible gill-current. In 

 general, it draws water through an opening at the 



