248 



COMPARATIVE ANATOMY. 



InPetromyzon the individual branchial sacs open freely to the 

 exterior; but in Myxine this original condition becomes modified 



FIG. 200. DIAGRAM OF A LONGITUDINAL SECTION THROUGH THE HEAD OF 

 Ammocoetcs (A) AND Petromyzon (B). 



Cli K 



FIG. 201. LONGITUDINAL SECTION THROUGH THE HEAD OF Ammocve-tes. 



V, velum ; P, papillae of mucous membrane ; K, K, K, three anterior gills ; Th, thyroid 

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

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

 Jnf, 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 come 

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

 in these Fishes they consist of closely-approximated transverse 

 Iamina3 (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. 



