580 



ZOOLOGY 



SECT. 



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 the 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 

 functions as a true lung. Probably the inner surface of the gill- 

 cover or branchiostegite performs a respiratory function in the 

 Crayfishes. 



In Amphipoda, also, the gills (Fig. 478, br) are outgrowths of 

 the thoracic limbs : in Isopods they are the modified endopodites 

 of the second to the fifth pleopods ; in some of the terrestrial 



FIG. 479. Anterior portion of Euphausia pellucida. a\, antennule ; 02, antenna; 



first abdominal segment ; au, eye ; br.i br. 8> podobranchiae ; cth. cephalothorax ; en.i, en.z, 

 endopodites of first two thoracic limbs ; ex.i ex. 6 , exopodites of first six thoracic limbs ; 

 h. heart ; /, digestive gland; m, "stomach " ; ov. ovary ; ovd. oviduct ; I VIII, proto- 

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



