The rate of "breathing" varies considerably in different Fishes, even in allied species.[303] In the Blue Wrasse (_Labrus_), and the Rockling (_Motella_), the number of respirations per minute is 15, in the Minnow (_Leuciscus_), and Stickleback (_Gastrosteus_), as many as 150. A deficiency of oxygen in the water accelerates the respiratory movements, and the Fish appears to "pant" or breathe hurriedly. In the Lampreys, both inspiration and expiration may take place through the external gill-apertures by the alternate expansion and contraction of the gill-sacs, more especially when the suctorial buccal funnel is used for the attachment of the animal. On the other hand, the singular habits of the Myxinoids involve a further modification of the respiratory process. In these Cyclostomata the inspiratory current enters the external naso-pituitary aperture and reaches the pharynx through the naso-pituitary canal, and thence, as an expiratory stream, traverses the gill-sacs on its way outwards. The pharynx is closed behind the last pair of gill-sacs by a constrictor muscle, which prevents the entrance of the water into the oesophagus, and converts the pharynx into a respiratory tube for the time {289}being; but, when food is being swallowed, the pharyngeal constrictor 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 the fact that in _Periophthalmus_ the tail is used for respiration. Hickson[304] 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[305] subsequently pointed out, through the agency of its extremely vascular caudal fin. [Illustration: FIG. 167.—Embryos of the Electric Torpedo (_Torpedo ocellata_). A, dorsal view; B, ventral view of a slightly younger specimen. _cl_, Cloaca; _el.o_, electric organ; _ex.b_, external gills; _p.f_, pectoral fin; _pv.f_, pelvic fin; _sp_, spiracle; _y.s_, stalk of yolk-sac.] 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. {290}[Illustration: FIG. 168.—Head of young _Polypterus_. _ex.g_, External gill of the left side. (From Steindachner.)]