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two sets of setae probably strain the water as it passes 

 through to the branchial chamber. 



At its anterior end the branchial chamber is 

 extremely shallow on account of the roof sloping down at 

 a considerable angle. Above the branchial ridge, already 

 referred to, there is an extremely narrow cavity between 

 the top of the ridge and the roof of the chamber. This 

 cavity is continued forward and inward into the pre- 

 branchial chamber. 



P r e - b r a n c h i a 1 chambers. One pair. 

 They are situated at the side of the mouth, each being in 

 front of and connected with the branchial chamber of the 

 same side. Each chamber is produced by the ingrowth of 

 the inner edge of the anterior part of the sub-branchial 

 region of the carapace. Its walls, therefore, are con- 

 tinuous with those of the branchial chamber. The pre- 

 branchial chamber is much smaller and shallower than the 

 branchial chamber. On its anterior and inner side it is 

 connected with the exterior by means of a wide opening 

 the exhalent aperture. 



The current through the branchial and pre-branchial 

 chambers is caused by the vigorous action of the seapho- 

 gnathite. The latter lies in the pre-branchial chamber, 

 and when at rest the anterior surface faces upward. 

 Normally the scaphognathite displays the following 

 movements: — The action of the extensor muscles tends to 

 pull the ventral surface backward. This is followed by a 

 sharp forward blow of the outer lobe of the scapho- 

 gnathite, caused by the action of the outer flexors. This 

 is immediately followed by an undulating movement of 

 the inner lobe, caused by the accessory muscles and inner 

 flexors. In this way the water is baled out of the exhalent 

 aperture. This current from behind forward is probably 

 assisted by the energetic action of the exopoditic flagella 



