. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. 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 fir
. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. 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, : 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
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