AQUATIC RESPIRATION OF FISHES 11 



The mechanisms ventilating the gill lamellae contained within 

 the pouches are different in the two groups. Lampreys appear to 

 remain attached to their prey for longer periods than do hag- 

 fishes which have more of a scavenging existence, and their 

 ventilation mechanism is well adapted to this situation. As the 

 mouth is fixed to the prey by a suction mechanism, water cannot 

 enter this way and consequently in lampreys the respiratory 

 current passes through the external gill openings during both 

 phases of the respiratory cycle. Expiration is an active process 

 which results in constriction of the branchial region and the 

 ejection of water from the individual pouches. The elastic recoil 

 (fig. 4a) of the visceral arch skeleton produces a reduced pressure 

 within the pouches which rises again as water enters them. The 

 path of the water in and out of the external openings is not 

 exactly the same, for they are guarded by valves which ensure 

 that water passes through the gill lamellae only during the ex- 

 piratory phase. 



Recent work suggests that ventilation in myxinoids is com- 

 pletely diff*erent from that in lampreys despite the accounts given 

 in many standard text-books. The mechanism appears to be the 

 same both in free-living stages when the animal burrows in mud 

 and during the relatively brief (four- to five-minute) periods 

 when the animal 'burrows' into its prey. Water is drawn in 

 through the nostrils and passes out through the external gill 

 openings. The propulsion of this current is due to the velum 

 contained in a special anterior chamber of the alimentary canal 

 (fig. la). X-ray cine films with contrast media in the respiratory 

 chambers have confirmed this function, and have also revealed 

 that parts of the respiratory tract may contract and assist the 

 ventilation. There are also sphincters in the gill ducts on both 

 sides of the pouches. It appears that these aid in the directional 

 propulsion of the current but are also important as filters to 

 prevent smothering of the gills. The frequency of the respiratory 

 movements of Myxine is about 1 1-15 per minute. 



In addition to the gill surfaces, a great part of the gaseous 

 exchange of cyclostomes occurs through the skin and this is 

 probably sufficieiit to satisfy the needs of Myxine during feeding. 



