127 



The inner wall is well calcified, and is formed by the 

 thoracic epimera (" flancs "). The gills rest on the inner 

 wall. 



The development of the branchial chambers would 

 lend to produce a stagnant layer of water around the 

 respiratory organs. But the nature of their function 

 requires that the gills should be in contact with water 

 containing a normal amount of dissolved oxygen. So 

 that, correlated with the formation of the branchial 

 chamber, an arrangement has been effected for producing 

 a constant stream of water over the gills. This necessi- 

 tates two things (1) inhalent and exhalent openings in 

 connection with each branchial chamber, and (2) some 

 mechanism for producing the current of water through 

 the chambers. 



In the Macrura the inhalent opening is situated 

 between the inner edge of the branchiostegite and the 

 base of the thorax. In Cancer, however, this opening is 

 considerably smaller. Between the chela and the last 

 pereiopod the sub-branchial region is closely applied to 

 the base of the thorax, and the line of separation between 

 the two is guarded by a thick growth of long setae, so 

 that it is highly improbable that any water can gain 

 entrance to the branchial chamber in this region. Above 

 the last walking leg, however, there is a small slit opening 

 into the posterior region of the branchial cavity. This 

 is the posterior inhalent aperture. In front of the 

 coxopodite of the chela there is a well-defined transverse 

 opening leading into the anterior part of the branchial 

 cavity. This is the anterior inhalent aperture. The 

 latter is guarded in front by the coxa and flabellum of the 

 third maxillipede, which, on their posterior borders, are 

 clothed with long setae. There are also numerous setae 

 on the anterior face of the coxopodite of the chela. These 



