THE BRANCHIAL CHAMBER AND THE GILLS. 25 



fully introduced into the cleft from behind, as high up 

 as it will go without tearing anything, and a cut is then 

 made, parallel with the middle line, as far as the cervical 

 groove, and thence following the cervical groove to the 

 base of the outer foot-jaws, a large flap will be removed. 

 This flap of the carapace is called the branchiostegite 

 (fig. 1, bg), because it covers the gills or branchiae 

 (fig. 4), which are now exposed. They have the appear- 

 ance of a number of delicate plumes, which take a direc- 

 tion from the bases of the legs upwards and forwards 

 behind, upwards and backwards in front, their summits 

 converging towards the upper end of the cavity in which 

 they are placed, and which is called the branchial 

 chamber. These branchiae are the respiratory organs ; 

 and they perform the same functions as the gills of a 

 fish, to which they present some similarity. 



. If the gills are cleared away, it is seen that the branchial 

 cavity is bounded, on the inner side, by a sloping wall, 

 formed by a delicate, but more or less calcified layer of 

 the exoskeleton, which constitutes the proper outer wall 

 of the thorax. At the upper limit of the branchial cavity, 

 the layer of exoskeleton is very thin, and turning out- 

 wards, is continued into the inner wall or lining of the 

 branchiostegite, which is also very thin (see fig. 15, p. 70). 

 Thus the branchial chamber is altogether outside the 

 body, to which it stands in somewhat the same relation 

 as the space between the flaps of a man's coat and his 

 waistcoat would do to the part of the body enclosed by the 



