PHYLUM ARTHROPODA 



549 



jspiratory organs, but other Schizopods possess tufted podobranchise 

 (Fig. 436) quite uncovered by the carapace. In the Decapoda the 

 gills may be either plume-like, as in Astacus and its allies, or the 

 delicate cylindrical gill-filaments may be replaced by flat plates, as 

 in Crabs and many Prawns. It is in this order only that we find 



436. Anterior portion of Euphausia pellucida, a Schizopod. a\, aiitennule ; ant.%, an- 

 tenna ; aii.l, first abdominal segment ; au, eye ; br.l 8, podobranchia ; cth. cephalo thorax ; en.l, 

 en.3, endopodites of first two thoracic limbs ; en.l en.6, exopodites of first six thoracic 

 limbs ; h. heart ; /, digestive gland ; -m, stomach ; or. ovary ; onl. oviduct ; I VIII, protopo- 

 dites of thoracic limbs. (From Lang's Comparative Anatomy.) 



the three types of gill described in Astacus and the examination 

 of numerous forms leads to the conclusion that the typical or 

 eoretical branchial formula for the group is as follows : 



the. 



Actually, however, this formula never occii^, as there is always 

 more or less reduction in the number of gills. Palinurus has the 

 highest number known, viz., twenty-one, and in the Common Crab 

 the total number is nine. ^p 



Many Crabs live on land, and their gills are enabled to discharge 

 their function in virtue of the moisture retained in the nearly 

 closed gill-chamber. In the Cocoa-nut Crab (Birgus) the upper 





