366 THE INVERTEBRATA 



In the hinder region the two layers are not very widely separated, 

 and there are in this space only blood channels and connective tissue, 

 but anteriorly branches of the liver and gonad intrude there. The 

 edge of the branchiostegite fits close against the flank of the thorax 

 and the exopodites of the maxillipeds, leaving however the following 

 openings: (i) small slits, one above each leg, (2) a large opening in 

 front of the coxopodite of the chela, (3) a still larger opening in front 

 of the mouth. These openings lead to and from the gill chamber. In 

 the flattening of the body, the lateral wall of the thorax has come to 

 face in great part upwards, so that the gills instead of being directed 

 vertically from their attachments, are directed more or less hori- 

 zontally inwards over the convex, mound-like inner wall of the gill 

 chamber. The gills are of the kind known asphyllobranchiae. That is, 

 the axis of each, instead of bearing filaments as in the gills of the cray- 

 fish (trichobranchiae), has on either side a row of plates, set close like 

 the leaves of a book. The podobranchiae stand out from the base of 

 an epipodite, which bears also a slender process known as a mastigo- 

 branchia. In the crayfish the gill lies along this and is fused with it 

 (Fig. 272). The first maxilliped has a mastigobranchia without a 

 podobranchia. The gill series of Carcinus is shown in the following 

 table : 



The mastigobranchiae lie in the gill chamber, that of the first 

 maxilliped in the epibranchial space above (external to) the gills and 

 those of the second and third maxillipeds in the hypobranchial space 

 below the gills. Their function is the cleaning of the gills. 



In front, the gill chamber narrows to an exhalant passage, which 

 contains the scaphognathite and leads to the large anterior opening. 

 The scaphognathite, working to and fro, drives water out of this 

 opening and so draws in a current through the other apertures. The 

 opening in front of the chela can be closed by a flange on the coxopo- 

 dite of the third maxilliped, and so the current can be regulated. The 



