2-48 



COMPARATIVE ANATOMY. 



In Petromyzon the individual branchial sacs open freely to the 

 exterior; but in Myxine this original condition becomes modified 



FG. 200. DIAGRAM <>F A LONGITUDINAL SECTION THROUGH THE HEAD OF 

 Ammoccetcs (A) AND Petromyzon (B). 



T.n JnffflML 



1 J o r 



fit Ji 



N 



FIG. 201. LONGITUDINAL SECTION THROUGH THE HEAD OF Animocu-/i'x. 



/', velum ; P, papilhe of mucous membrane ; K,K,K, three, anterior gills ; Th, tliymid 

 gland (hypobranchial furrow) ; N, nasal sac ; *, communication between the 

 ventricle of the olfactory lobe and that of the prosencephalon ; Ep, epiphysis ; 

 Jtif, infundibulum ; HH, metencephalon ; ML, medulla oblongata ; b, c, 

 ventricles of the mid- and hind-brain ; o, subdural cavity ; Ch, notochord ; 

 R, spinal cord. 



by the external gill-passages growing out into long tubes, which 

 unite to form a common duct on either side ; this opens far behind 

 the branchial apparatus on the ventral side of the body. 



Fishes. From the Elasmobranchii onwards the gills conic 

 into close relation with the skeletal part of the visceral arches, and 

 in these Fishes they consist of closely-approximated transverse 

 lamina? (Fig. 202), which are firmly attached to both sides of the 

 septa which separate the individual gill-sacs from one another, so 

 that each septum bears gill-laminae on both its anterior and pos- 

 terior surface. The gill-sacs, of which there are usually five (six 

 or seven in Notidanus), usually open separately to the exterior. 



