5 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, 



and the circulation is conse- 

 quently nmch modified (Hyrtl 

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

 dorsal aorta is formed b}^ 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 Fsociformes 

 [42]). 



Fig. 404. 



(Boulenger 



Diagram of the circulation in Monopterus 



javanensis, Lac. The right kidney and its veins 



have been removed ; the right jugular vein is 



incomplete. Arrows show tin 1 direction of the 



blood-stream. White vessels contain arterial 



blood, .shaded vessels mixed, ami black vessels 



venous blood. I-IV, four gill-arches : .I.e. 



coeliac artery ; A.d, dorsal aorta ; BA, bulbus : 



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



ductus Cuvieri ; L, liver ; N, kidney ; l:,A = IV, 



radix aorta: I', ventricle; V.c, cardinal vein ; 



V.h, hepatic vein; I'.jrf, right jugular vein; 



Family SvMBRAXCHIDAE. The r ->S left jugular vein; !>. portal vein; ;,. 



,.,.-,! t afferent vessel of third gill-arch. (From W. 



post-temporal is still present and volz.) 



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 



