1 8 AQUATIC BREATHING 



In the singular group of lampreys and slime eels (Mar- 

 sipobranchs, v. p. 57), the segmental arrangement of the 

 gills seems of a primitive pattern. In the Californian 

 Myxinoid (p. 59) the slits are as numerous as thirteen and 

 fourteen on either side, each opening directly from the 

 gullet to the neck surface (Fig. 9, G, *, BS'., BD). In the 

 lamprey the conditions are similar, but the number of gill 

 slits is reduced to seven. In Myxine (Fig. 10, G, BS', BD, 

 BO} the outer portions of the canals becoming produced 

 tail-ward have merged in a single pore (Fig. 71 *). In these 

 forms each gill canal has become dilated at one point of 

 its course, and in this sac-like portion the blood-suffused 

 tissues have grouped themselves into leaf-like plates (gill 

 filaments, or lamellae, BS') to increase their surface of 

 contact with the out-passing water. The dilating power 

 of this gill sac has then become specialized so that even 

 should the animal's mouth be closed, water for respiration 

 could be drawn in through the canal's outer opening: 

 from this acquired function the elaboration of bran- 

 chial muscles and a supporting framework of cartilage 

 (branchial . basket, Fig. 69^, BB) may have taken its 



origin. 



Among fishes proper many stages in the evolution of 

 gill organs are represented. They show altogether a 

 marked advance over the conditions of Fig. 9. There 

 has been a general tendency to press closely together the 

 gill pouches and to elaborate into thinner and larger 

 lamellae the blood-suffused tissue. In this process the 

 gill chamber has become slit-like, bearing gill lamellae only 

 on its front and rear margins ; its supporting tissue has con- 

 solidated into stout vertical gill bars, the gill structures in 

 general, becoming more highly perfected, tending to recede 

 from the surface. These conditions may best be illustrated 



