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- 
tinually 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 scaphognathite), which is con- 
‘eave 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. 
