VASCULAR SYSTEM OF FISHES. 263 



and the last or pharyngeal arch beset Avith teeth only. In the Re- 

 mora and many other Fishes, the defensive tubercles on opposite sides 

 of the same branchial fissure interlock, like the teeth of a cog-wlicel. 

 In the Lepidosiren annectens, or Protopterns, short valvular pi'o- 

 cesses are developed from the sides of those branchial fissures only 

 which lead to the gills, the first and second arches having no gills. 

 In the Conger, all the branchial arches are devoid of defensive fringes 

 or tubercles. 



The immediate force of the heart's contraction is applied by a 

 short and rapidly divided arterial trunk upon the branchial circula- 

 tion. Only in a few fishes is the heart removed backwards from the 

 close proximity of the gills, and then the branchial artery is pro- 

 portionally elongated ; as in the Eel tribe, especially the Synhran- 

 chidcB : the artery is long in the Planirostra {fig- 61. «). The primary 

 branches are always opposite and symmetrical, but vary in number in 

 different species. Very commonly, as in the Perch, they are three in 

 number on each side ; the first branch dividing to supply the fourth and 

 third gills, the second going to the second, and the third to the first gill. 

 In the Polypterus and Skate there are only two primary branches of 

 each side : the first supplies the three posterior gills ; the second, 

 formed by a terminal bifurcation of the branchial trunk, supplies the 

 anterior gill in the Polypterus, and in the Skate bifurcates to supply 

 also the uniserial, opercular, or hyoid gill. The Fox-Shark [Alopias) 

 and the Lcpidosteus {fi'ff. 70.) give examples of four pairs of primary 

 branches from the branchial trunk. In the Shark the first pair come 

 off" close together from the dorsal part of the trunk : the arteries of 

 the last pair quickly bifurcate, and thus each of the five branchial 

 fissures receives its artery. We saw in the Myxinoids the excep- 

 tional instances of the bifurcation of the branchial trunk by a vertical 

 division into two lateral forks, extended in one species to near its 

 base : the Lepidosteus presents the still rarer example of the trunk 

 being cleft liorizontally into an upper and lower primary division ; 

 the upper or dorsal division sends off two branches on each side, the 

 posterior dividing to supply the fourth (fiff. 70, 5) and third (ib. 4) 

 gills, the anterior going to the second gill (ib, 3) : the lower division 

 sends off" the pair of arteries to the first pair of gills (ib. 2), then 

 extends forward and bifurcates to supply the uniserial opercular 

 gills {fiff. 1.), which are present in this ganoid genus, as in the 

 Sturgeon.* In all osseous fishes the artery of each biserial pec- 

 tinated gill extends along the grooved convexity of the branchial 

 arch, between the bases of tlie processes, exterior to the vein : it 

 gives off" two branches opposite each })air of processes, which pass 



* S'jc I'rof. ^liillcr's adinliable JMcinoir, xxv. 

 s 4 



