80 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



covered with rapidly vibrating filaments, or cilia, by 

 means of which a current of water is kept con- 

 tinualty flowing over the gills, but there are none of these 

 in the crayfish. The same object is attained, however, in 

 another way. The anterior boundary of the branchial 

 chamber corresponds with the cervical groove, which, as 

 has been seen, curves downwards and then forwards, 

 until it terminates at the sides of the space occupied by 

 the jaws. If the branchiostegite is cut away along the 

 groove, it will be found that it is attached to the sides of 

 the head, which project a little beyond the anterior part 

 of the thorax, so that there is a depression behind the 

 sides of the head — just as there is a depression, behind a 

 man's jaw, at the sides of the neck. Between this 

 depression in front, the walls of the thorax internally, 

 the branchiostegite externally, and the bases of the for- 

 ceps and external foot-jaws below, a curved canal is in- 

 cluded, by which the branchial cavity opens forwards as by 

 a funnel. Attached to the base of the second maxilla 

 there is • a wide curved plate (fig. 4, 6) which fits 

 against the projection of the head, as a shirt collar might 

 do, to carry out our previous comparison ; and this scoop- 

 shaped plate (termed the scaj^hognathite) , which is con- 

 cave forwards and convex backwards, can be readily 

 moved backwards and forwards. 



If a living crayfish is taken out of the water, it will 

 be found that, as the water drains away from the branchial 

 cavity, bubbles of air are forced out of its anterior opening. 



