142 THE BIOLOGY OF MARINE ANIMALS 



chambers. In lampreys the gill filaments are situated in branchial pockets 

 which open to the exterior by separate orifices; water is pumped in and 

 out of the branchial chambers through these openings. In Myxine the 

 situation is peculiar in that the pouches lead into longitudinal canals which 

 open to the exterior by a common aperture some distance behind the 

 branchial region. The hagfish bores into its prey, and under these condi- 

 tions the respiratory current enters the dorsal nasopituitary aperture and 

 passes through the gill sacs, to be expelled posteriorly. Separate external 

 gill apertures are present in selachians. When resting on the bottom, rays 

 and skates draw in a respiratory current through the spiracles, which are 

 guarded by valves, and expire through the gill openings in the normal 

 manner. 



The branchial chamber in teleosts is covered by an operculum. Within, 

 the gill filaments of neighbouring arches meet in such a way that all the 

 respiratory current must pass over the branchial lamellae (Fig. 4.3). 

 Gaseous exchange is facilitated by the counter-current principle by which 

 the blood flows through the lamellae in a direction opposite to that taken 

 by the water passing outside. Consequently, the blood leaving the lamellae 

 comes into equilibrium with the water entering the gills. The respiratory 

 mechanism is such that water flows continuously over the gills. Inspiration 

 is produced by dilatation of the buccal cavity and branchial chambers, 

 while the opercula close and the mouth opens. During expiration the 

 mouth and buccal valves are closed, and the opercula open; water is 

 passed along the gills by reduction of the buccal cavity. In coughing- 

 reflexes the tips of the filaments are separated by contraction of adductor 

 muscles, and accumulated material is ejected. 



Variations from type in the structure and functioning of the branchial 

 apparatus are numerous. Very active pelagic fish, notably the mackerel, 

 do not make respiratory movements; rather, ventilation is achieved by 

 swimming through the water with open mouths. Indeed, these animals are 

 condemned to ceaseless locomotion to obtain the oxygen necessary for 

 their metabolic requirements (9, 33). 



Estimates of branchial respiratory areas in selachians give values of 

 0-7 cm 2 /g body weight in Cetorhinus and 1-8 cm 2 for Scyliorhinus. Values 

 for teleosts range from 1 to 18 cm 2 /g body weight. Active fast-swimming 

 fishes, such as menhaden (Brevoortia tyrannus) and mackerel {Scomber 

 scombrus) have relatively much greater gill areas than sluggish benthic 

 species such as toadfish {Opsanus tau) and flounders {Pseudopleuroncetes 

 americanus). The respiratory area of the mackerel, for example, is more 

 than five times greater than that of the toadfish (1 1-58 and 1-97 cm 2 /g body 

 weight, respectively) (70, 103). 



Combined Feeding and Respiratory Gills. The extensive feeding- 

 filaments, branchial crowns and baskets of many marine animals are also 

 concerned with respiratory exchange. This is a necessary consequence of 

 their functional morphology, since they present large surfaces past which a 

 current of water is directed externally, while the blood stream circulates 



