CR USTA CEA GILLS 



329 



barbs on the shaft of a quill. Phyllobranchiffi and trichobranchise, between which 

 there are many transitory forms, are not found together in the same species. Tricho- 

 branchise are found in the Macrura (with the exception of the Scrgestidcc, dtrididce, 

 and the genera GcMa and Callianassa) ; phyllobranchue are found in all Ano'iimm 

 and Brachyura, and in those Macrura which do not possess trichobranchiie. 



Podobranchioe, arthrobranchire, and pleurobranchifs may occur together, even on 

 the same thoracic segment. They undergo many modifications and degenerations. 

 The arrangement of the branchial apparatus in the various Decapodan genera and 

 species may be given in branchial formulae. We shall here give the branchial 

 formulae of Astacus fluviatilis and of Cancer pagur us. 



Branchial Formula of Astaeus fluviatilis (Cray-fish), 



after Huxley. 



Branchial Formula of Cancer pagurus (after Huxley). 



While in other Dccapoda the water enters the respiratory cavity through the 

 lower longitudinal slit, in the Bracliyura the water passes in and out only through 

 certain small apertures, which are variously placed. Many Brachyura and Aiwinurn 

 may live for a longer or shorter time or almost exclusive on land, and show various 

 adaptations which make it possible for them to retain water in the respiratory cavity, 

 or to draw back into that cavity water which leaves it, or to breathe air direct. It 

 would lead us too far to describe all these adaptations in detail. We shall only 

 consider the respiratory organs of Birgus latro (Anomura), which lives in holes in the 

 earth (Fig. 229). The respiratory cavity of this animal falls into two parts, an 



