MODIFICATIONS OF STEDCTURE IN DECAPOD CRUSTACEA. 215 



remaining Oxystomata the exlialant orifices have been re- 

 garded as situated at the base of the chelipeds, i. e. as occupy- 

 ing the normal position. 



It is perfectly true that the proximal aperture by which 

 water enters the branchial chamber does occupy this position; 

 but an examination of the crabs Calappa and Matuta has 

 revealed to me that before the water enters the branchial 

 chamber by this aperture, it has in all probability previously 

 traversed certain accessory channels formed by the apposition 

 of the chelipeds to the inferior surface of the carapace. 



In the case of Calappa granulata it may be observed 

 that on each side of the median exhalaut orifices there exists 

 a well-marked slit-like aperture between the infra-orbital 

 margin of the carapace, and the serrated crests of the apposed 

 chelipeds (fig. 1, e). If one of the chelipeds be now with- 

 drawn from its flexed position, it may be noticed that this 

 slit-like aperture leads downwards and backwards as a well- 

 marked channel to the inhalant aperture at the base of the 

 cheliped. The channel is bounded internally by the ridge- 

 like outer edge of the third raaxilliped ; its other boundaries 

 are furnished by the approximated surfaces of the inner face 

 of the chelipeds, and the pterygostomial region of the cara- 

 pace. It is in fact an accessory channel formed by the 

 cheliped ; and the inhalant aperture is carried by its means to 

 the tip of the snout in precisely the same way that in the 

 Leucosiidae the corresponding aperture is carried forward by 

 means of the exopodite of the third raaxilliped. 



I propose to call this accessory water-channel the '^exo- 

 stegal canal." In its situation on the external side of the 

 pterygostomial portion of the branchiostegite it contrasts with 

 the more primitive branchial canals, which are endostegal in 

 position. It difl'ers from the characteristic aflferent canal of 

 the Leucosiidae in requiring the participation of the chelipeds 

 to complete its external wall, whereas in the Leucosiidae the 

 third maxillipeds are alone concerned in bringing about the 

 same result. 



Attention may now be directed again to fig. 1, which shows 



