VASCULAR SYSTEJI OF FISHES. 



259 



sac of a IMyxine {fig. 66. ni) to be split througli its plane, and each 

 half to be glued by its outer smooth side to an intermediate septum, 

 which would then support the opposite halves of two distinct sacs, and 

 expose their vascular mucous surface to view. Produce these vascular 

 surfaces into lamellae, pectinated processes, tufts or filaments, proceed- 



G'7 



■'■/V J ' 



^: 



M?*iB»*.i" 



Two gill-sacs, Bdello- 

 stoma. 



Two sill-sacs, Lamprey. 



ing from an intermediate arch or basis of support, and you have the 

 gill of an ordinary osseous or cartilaginous fish. Such a gill is the 

 homologue, not of a single gill-sac, but of the contiguous halves ot 

 two distinct gill-sacs, in the Myxines. Already, in the Lampreys, the 

 first stage of this bi-partition may be seen (fig. 67. m), and conse- 

 quently in these Dermopteri, as in all higher fishes, a different ar- 

 tery 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. Before describing the branchial 

 vessels it Avill 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 integuments 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 it enters from the pharynx (ib. fi) : these are 

 called ' fixed gills,' and the species possessing them are characterised 

 as ' pisccs branchiis fixis.' In all Osseous, Plec- 

 tognathic, Lophobranchiate, Ganoid, and IIolo- 

 cephalous fishes the outer border of the sup- 

 porting branchial arch is unattached to the skin, 

 and plays freely backwards and forwards, with its 

 gill-sui-faces, in a common gill-cavity which has a 

 single outlet, usually in the form of a vertical fis- 

 sure : the species with this structure are called 

 ' pisces branchiis liberis.' In the Myxine the outlets 

 of the six lateral branchial sacs (fi'g. 68. m)on each 

 side are produced into short tubes, which open into 

 a longitudinal canal (k), directed backwards, and 

 discharging the branchial stream by an orifice (h) near tlie middle 



Branchial organs, 

 Myxine. 



