288 FISHES CHAP. 
olyodon also constitute an efficient filter, and the same may be 
said of several plankton-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 
“ breathing-vaives.” 
The rate of “breathing” varies considerably in different 
Fishes, even in allied species.” In the Blue Wrasse (Labrus), 
and the Rockling (JMotella), 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. 
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