GILLS OF FISHES. 



477 



the branchial stream by an orifice, h, near the middle line of the 

 ventral surface : between the two outlets of these lateral longi- 

 tudinal canals, but nearer the left one, is a third larger opening, 

 i, which communicates by a short duct with the end of the long 

 oesophagus, I, and admits the water, which passes from that tube 

 by the lateral orifices,/, leading into the branchial sacs. This is 

 the first step in developement beyond that simpler condition which 

 prevails in the Lancelet, where the whole parietes of a much 

 dilated oesophagus, fig. 169, rr, are organised for respiration ; and 

 besides the pharyngeal opening, ph, the sac communicates by a 

 short and wide ' ductus oesophago-cutaneus,' ib. od, with the 

 external surface, and also with the peritoneal cavity. The 

 common respiratory surface of the oesophagus is ciliated in the 

 Lancelet. The sacs developed from the oesophagus, and specially 

 set apart for respiration in the Myxinoids, have a highly vascular, 

 but not a ciliated mucous surface : this is disposed in radiated 

 folds, and is further increased by secondary plicae. The seven 

 branchial sacs on each side of the oesophagus 

 have short external ducts, fig. 313, k, which 

 open by as many distinct orifices in the skin 

 in a species of Bdellostoma hence called hep- 

 tatrema : the internal branchial ducts com- 

 municate by as many openings, ib. f, with 

 the oesophagus. In the Lampreys there are, 

 also, seven stigmata on each side ; but another 

 stage in the separation of the respiratory from 

 the digestive tract is here seen, for each in- 

 ternal duct communicates with a median 

 canal, fig. 310, d, beneath and distinct from 

 the oesophagus, terminating in a blind end 

 behind, and communicating anteriorly with 

 the fauces by an opening guarded by a double 

 membranous valve. 



In all higher fishes the inlets to the branchial 

 interspaces are situated on each side the 

 fauces, and are equal in number with those 

 interspaces, fig. 316, l — 5. The outlets are, 

 with the exception of the Plagiostomes, single 

 on each side : they vary much in size ; are 

 relatively largest in the Herring and Mackerel 

 families, smallest in the Eels and Lophioid 

 fishes ; in some of the small Frog-fishes, 

 Antennarius, the circular branchial pore is produced into a 



Branchial slits and lungs, 

 Lepidosiren. sxxin. 



