1. Outline of the hag, showing: the two ventral openings (h) by which the water escapes from the gills. 2. Anterior portion of the body laid open on the ventral side to show the branchial sacs. æ, gullet laid open, showing on each side the six apertures by which the water is conducted to the corresponding branchial pouches (g); d, one of the efferent ducts by which the water is conducted away from the branchial pouches; cd, common efferent duct on the left side, opening on the ventral surface by a branchial pore (p or h of Fig. 1); gd, special canal developed on the left side only, and leading from the gullet into the common branchial duct; n, unpaired nostril, with its “barbels;” m, mouth.