Pigs. 9-12. Arrangement of gills of Bdellostoma (9), Myxine (10), Shark (n), and 

 Teleost (12). In each figure the surface of the head region is shown at the left. 



B. Barbels. BD. Outer duct from gill chamber, BS. BO. Common opening of outer 

 ducts from gill chambers. BS. Branchial sac, or gill chamber. BS". Branchial sac, sec- 

 tioned so as to show the folds of its lining membrane. G. Lining membrane of gullet. 

 GB. Gill bar, supporting vessels and filaments of gills. GC. Outer opening of gill cleft. 

 GF. Gill filament. GR. Gill rakers. GV. Vessels of gill. J t J'. Upper and lower jaw. 

 M. Mouth opening. N,N'. Anterior and posterior opening of nasal chamber. OP. Oper- 

 culum. SP. Spiracle. ST. Tendinous septum between anterior and posterior gill filaments. 

 * Denotes the inner branchial opening; -, the direction of the water current. 



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