276 



COMPARATIVE ANATOMY 



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

 the branchial apparatus on the ventral side of the body. 



EiG. 220. — Diagram of a LoNGITUDI^"AI. Section theougf the Head of 

 THE Labval (A) AND ADULT (B) Peiromyzon. 



Ep JnfmML 



Ch. It 



Fig. 221. — Longitudinal Section thkough the Head of an Ammoccete. 



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

 thyroid (hjrpobranchial furrow) ; N, nasal sac ; *, communication be- 

 tween the ventricle of the olfactory lobe and that of the prosencephalon ; 

 Ep, epiphysis ; Jnf, infundibulum ; HH, meteneephalon ; 211, medulla 

 oblongata ; h, c, ventricles of the mid- and hind-brain ; o, subdural cavity ; 

 Ch, notochord ; R, spinal cord. 



Fishes. — From the Elasmobranchii onwards, the gills are in 

 close relation with the visceral skeleton, and in these Fishes they 

 consist of closely-approximated transverse laminae (Figs. 222 A, 223), 

 which are firmly attached to both sides of the septa which separate 

 the individual gill-sacs from one another, so that each septum bears a 

 half-gill, or hemibranch, on both its anterior and posterior surface. 

 A gill, or holobranch, thus consists of the branchial arch plus the 

 posterior hemibranch of the sac in front of it and the anterior 

 hemibranch of the following sac. The gill-sacs, of which there are 

 commonly five,^ open separately to the exterior, and a rudimentary 



^ There are six in Hexanchus and Chlamydoselaohe and seven in Heptanchus. 



