476 



ANATOMY OF VERTEBRATES. 



its entire margin, the condition of the plagiostomous gill is effected. 

 If the septum be liberated at the outer part of its circumference 

 and the vascular surfaces are produced into pectinated lamelli- 

 gerous processes, tufts, or filaments, proceeding from the free arch, 

 the gill of an ordinary osseous or teleostomous fish is formed. 

 Such a gill is the homologue, not of a single gill-sac, but of the 

 contiguous halves of two distinct gill-sacs, in the Myxines. 

 Already, in the Lampreys, the first stage of this bi-partition may 

 be seen, fig. 314, m, and the next stage in the Sharks and Rays: 

 consequently in these fishes, a different artery goes to the anterior 

 branchial surface of each sac or fissure from that which supplies 

 the posterior branchial surface of the same fissure ; whilst one 

 branchial artery is appropriated to each supporting septum or 

 arch between the fissures, as it is to the liberated septum or 

 branchial arch in the Teleostomi. 1 Before describing the branchial 



o 



vessels it will be necessary to describe the organs upon which 



they ramify. 



In the Lampreys and Plagiostomes each supporting septum of 



the two (anterior and posterior) branchial mucous surfaces is 



attached to the pharyngeal and dermal integu- 

 ments by its entire peripheral margin, and the 

 streams of water flow out by as many fissures 

 in the skin, ib. k, as those by which they enter 

 from the pharynx, ib./: these are called ( fixed 

 gills,' and the species possessing them are cha- 

 racterised as 'pisces branchiis fixis.' In the 

 Teleostomi = Osseous, Plectognathic, Lopho- 

 branchiate, Ganoid, and Holocephalous fishes, 

 the outer border of the supporting branchial 

 arch is unattached to the skin, and plays freely 

 backward and forward, with its gill-surfaces, 

 in a common gill-cavity which has a single 

 outlet, usually in the form of a vertical fissure : 



the species with this structure are called ( pisces branchiis 



liberis.' 



In the Myxine the outlets of the six lateral branchial sacs, fig. 

 315, m, on each side are produced into short tubes, which open 

 into a longitudinal canal, k, directed backward, and discharging 



1 CXLV. p. 258. Prof. Milne Edwards has exemplified this homc-lo^y by the sub- 

 joined formula -. 



Osseous Fishes B . ac . B 1 B2 B.3 B4 



315 



Branchial organs, 

 Myxiue 



Plagiostomous Fishes 



b.b 



b b 



B 1 B2 



b b 



B2 



b b 



b. 



B4 B 5 



