18 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 lamellz, AS') 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 A, B&B) 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 
lamellz the blood-suffused tissue. In this process the 
gill chamber has become slit-like, bearing gill lamellz 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 
