476 



ANATOMY OF VERTEBEATES. 



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 o'ill 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, vi, 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 sejitum or 

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

 l^ranchial arch in the Teleostomi} Before describing the branchial 

 vessels it will be necessary to describe the organs upon Avhicli 

 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 fiow out by as many fissures 

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

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

 gills,' and the sjiecies possessing them are cha- 

 racterised as ' p>isces branchiis fixis.' In the 

 Teleostomi = Osseous, Plectognathic, Lopho- 

 branchiate. Ganoid, and Holocephalous fishes, 

 the outer border of the supporting brancliial 

 arch is unattached to the skin, and plays freely 

 backward and forward, with its gill-surfirccs, 

 in a common gill-ca-\'ity which has a single 

 outlet, usually in the form of a vertical fissure : 

 with this structure are called ' pisccs branchiis 



Brancli ial organs, 



the species 

 liberis.' 



In tlic IMyxine 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, /(, directed backward, and discharging 

 Trof. Milne Edwards lias exemplified this homology hy the sub- 



' CXLV. p. 258. 

 joined formula ; — 



Osseons Fishes 



ria"iustomous Fishes 



h.hx h 



B 2 



h \h I \ h. 



