XI 



PHYLUM ARTHROPODA 



595 



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 the 

 three types of gill described in Astacus, and the examination 



FIG. 472. Anterior portion of Euphausia pellucida. j, antennule ; ant.-, antenna; ab.l, 

 first abdominal segment; uu, eye; br. 1 8, podobranchia ; cth. cephalothorax ; en.l, en.~', 

 endopodites of first two thoracic limbs; ex.1 ex.6, cxopodites of first six thoracic limbs; 

 h. heart ; I, digestive gland ; in, "stomach " ; ov. ovary ; ovd. oviduct ; / VIII, protopodites of 

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



of numerous forms leads to the conclusion that the typical or 

 theoretical branchial formula for the group is as follows : 



Actually, however, this formula never occurs, 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 only nine. 



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 

 part of the gill-chamber is separated from the rest and forms an 

 almost closed cavity into which vascular tufts project : it thus 



