288 FISHES CHAP. 



Polyodon also constitute an efficient filter, and the same may be 

 said of several plank ton -eating Teleosts. 



The Mechanism of Respiration. The aeration of the blood 

 is effected by the rhythmical suction of water into the oral 

 cavity, and its subsequent expulsion through the gill-clefts, 

 bathing the highly vascular gill-lamellae in its course. In any 

 single act of inspiration the mouth is opened, and the oral cavity 

 enlarged by the lateral expansion of its walls, When the oral 

 cavity is filled with water, the mouth is closed and the expiratory 

 process begins. By the lateral contraction of the oral walls the 

 water is driven outwards through the gill-clefts, and over the 

 gill-lamellae. During this process the branchial arches become 

 widely separated by the contraction of their muscles, the oper- 

 culum is elevated, and the oesophagus is closed by the contrac- 

 tion of its muscular wall. In many Fishes .the course of the 

 expiratory water-current is controlled by special valve-like folds 

 of the oral mucous membrane, the maxillary and mandibular 

 " breath ing- valves." ] 



The rate of " breathing " varies considerably in different 

 Fishes, even in allied species. 2 In the Blue Wrasse (Labrus\ 

 and the Eockling (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 inspira- 

 tion 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 



1 Dahlgren, Zool. Bull. ii. 3, Boston, 1898 ; Allis, Anat. Anz. xviii. 1900, 

 p. 257. 



2 M'Kendrick, Journ. Anat. and Phys. xiv. 1879, p. 461. 



