x RESPIRATORY ORGANS 289 
being; but, when food is being swallowed, the pharyngeal con- 
strictor is relaxed and the internal apertures of the gill-sacs 
are closed by the contraction of their own sphincter muscles. 
Tn addition to the usual respiratory organs it is probable that 
in not a few Fishes the superficial skin may share with the gills 
the function of breathing. In this connexion may be mentioned 
Fic. 167.—Embryos. of the Electric Torpedo (Torpedo ocellata). A, dorsal view ; 
B, ventral view of a slightly younger specimen. c/, Cloaca ; e/.o, electric organ ; 
ex.b, external gills; p.f, pectoral fin; pv.f, pelvic fin; sp, spiracle; y.s, stalk of 
yolk-sac. 
the fact that in Periophthalmus the tail is used for respiration. 
Hickson ' observed that a species of this genus, frequenting the 
extensive sandy shores of the Island of Celebes, often rests with 
its tail in the water, the head and trunk being exposed. Under 
such circumstances the gills are probably of little use, and the 
tail is utilised as a breathing organ, principally, as Haddon * 
subsequently pointed out, through the agency of its extremely 
vascular caudal fin. 
Some Fishes possess larval breathing organs; others, even 
when provided with gills, either utilise the air-bladder, or develop 
special accessory organs, for aquatic or, more usually, for aerial 
respiration. 
1 Naturalist in Celebes, London, 1889, p. 30. 2 Nature, xxxix. 1889, p. 285. 
VOL. VII U 
