CR USTA GE A— GILLS 



329 



barbs on the shaft of a quill. PhyllobranchiEe aud ti-ichobranchise, between which 

 there are many transitory forms, are not found together in the sama species. Trioho- 

 branchiae are found in the Macrura (with the exception of the Sergestidce, Carididce, 

 and the genera Gebia and CalUaimssa) ; phyllobranchise are found in all Anmnura 

 and Brachyura, and in those Maerura which do not possess trichobranchife. 



Podobranohife, arthrobranohife, and pleurobranchise may occur together, even on 

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

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

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

 formulae of Astacus fluviatilis and of Cancer pagurtcs. 



Branchial Formula of Astaeus fluviatilis (Cray-flsh), 

 after Huxley. 



Thoracic segments and limbs. 



A'l. 1st maxillipede 

 VII. 2d 

 VIII. 3d 

 IX. 1st ambulatory foot 



X. 2d 

 XL 3d 

 XII. 4th 

 XIII. 5th 



Podo- 

 branchiEe. 



Artlirobrancli i ee. 



I Anterior, i Posterior. 



Pleuro- 



, branohiffi. 



0(ep.) 



1 



1 



1 



1 



1 



1 







- ep. + 





















 Rudim. 

 Rudim. 



1 



Total. 



= (ep.) 

 z2 



= 3 + Rudim. 

 = 3 + Rudim. 

 = 1 



5+ [l + 2Ru. =18 + ep.+2R. 



Branchial Formula of Cancer pagurus (after Huxley). 



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

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

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

 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 



