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. 



In 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 



Fig. 167. — Embryos of the Electric Torpedo (Torpedo ocdlata). A, dorsal view ; 

 B, ventral view of a slightly younger specimen, el. Cloaca ; el.o, electric org.nn ; 

 ex.h, external gills ; p.f, pectoral fin ; pv.f, pelvic fin ; «p, 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. 



^ Naturalist in Celebes, London, 1889, p. 30. = Nature, xxxix. 1889, p. 285. 



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