S YMBRANCHIFORMES 



409 



The branchial openings have joined in a single ventral 

 opening (Fig. 405) ; the opercular bones, and especially the gills, 

 become reduced, respiration is carried on chiefly in the wall of 

 the buccal cavity and intestine, 



conse- 

 (Hyrtl 



and the circulation is 

 quently much modified 

 [233i], Volz [475]). The 

 dorsal aorta is formed by the 

 junction of the last pair of 

 aortic arches, which are complete 

 (Fig. 404) ; the jugulars return 

 the blood from the head and 

 other arches directly to the 

 heart. 



The scales are either minute 

 (Amphipnoidae) or altogether 

 absent (Symbranchidae). Dermal 

 folds without lepidotrichia alone 

 represent the vestiges of the 

 median fins ; and the paired fins 

 have been lost, leaving only a 

 slender pectoral girdle, attached 

 to the skull in the Symbranchi- 

 dae, but free in the Amphipnoi- 

 dae. 



Unknown in the fossil state, 

 these peculiar fish, which have 

 now been separated from the 

 Anguilliformes and placed in a 

 special sub -order, may have 

 been derived from some primi- 

 tive group which had lost 

 the mesocoracoid arch, or 

 perhaps even from some 

 member of the Clupeiformes 

 or Esociformes (Boulenger 

 [42]). 



t---Acf 



FIG. 404. 



Diagram of the circulation in Monopterus 

 /rf -;.s, Lac. The right kidney and its veins 

 have been removed ; the right jugular vein is 

 incomplete. Arrows show the direction of the 

 blood-stream. White vessels contain arterial 

 blood, shaded vessels mixed, and black vessels 

 venous blood. I-IV, four gill-arches; A.c, 

 coeliac artery ; A.<!, dorsal aortii ; HA, bulbus : 

 c, carodid ; C.V, caudal vein; D, gut; D.C, 

 ductus Cnvieri ; L, liver ; X, kidney ; R.A = IV, 

 radix aorta; V, ventricle; V.r, cardinal vein; 

 I'.li, hepatic vein; V.jd, right jugular vein; 

 V.js, left jugular vein ; V.fi, portal vein ; x, 

 attrrent vessel of third gill-arch. (From W. 

 Volx.) 



Family SYMBRANCHIDAE. The 



, . .,, , 



post-temporal is still present and 



attached to the skull. Symbranchus 



has well-developed gills ; but Monopterus has only vestigial gill-lamellae 



on three arches. 



Symbranchus, Bl. (Fig. 405) ; America, E. Indies. Monopterus, Lac. ; 

 E. Indies and Archipelago. 



Family AMPHIPNOIDAE. The slender pectoral girdle is free, the post- 

 temporal having been lost. Gill-lamellae are found on the second arch 



