CRUSTACEA 



99 



epipodite. The arthrobranchs occur in pairs, on the joints 

 of the segments indicated. The four pleurobranchs are the 

 largest gills in the Norway lobster, but in the fresh-water 

 crayfish, for example, they are rudimentary, except the last. 

 The scaphognathite, or baler, the lamellar expansion of the 

 second maxilla, which lies in the cervical canal in front of the 

 gills, serves by its constant movement to draw a steady current 

 of water through the gill chamber. The current passes from 

 behind forwards, and thus brings about the respiratory 



Arthrobranchs 



Pleurobranchs 



Podobranch 

 Epipodite 

 ,5th pereiopod 



FIG. 47. Nephrops. The gills displayed by the removal of the 

 branchiostegite of the left side. 



exchange and the washing out of excretory products from the 

 gills. 



INTERNAL MORPHOLOGY. The integument consists of the 

 ectoderm and the chitinous cuticle which proceeds from it. 

 The ectoderm is supported on a basement membrane, beneath 

 which is a connective-tissue layer, bearing nerves and con- 

 taining pigment. The cuticle is hardened in segments by the 

 deposition of lime salts. It is uncalcified or soft at the joints, 

 on the inside of the gill covers and also on the gills themselves. 

 It may usually be resolved into four layers : (1) an outer, thin, 

 non-calcified layer ; (2) a calcified pigmented layer formed of 

 parallel lamellae ; (3) a calcified non- pigmented layer, usually 

 the thickest, also formed of parallel lamellae ; (4) a thin non- 

 calcified layer, also made up of lamellae. The cuticle arises 

 by the successive thickenings of the outer portion of the 

 ectodermal cells; this gives the lamellated structure, and 



