260 



LECTURE X. 



line of the ventral surface : between the two outlets of these lateral 

 longitudinal canals, but nearer the left one, is a third larger open- 

 ^^S (0' 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 (f) leading into the branchial sacs. 

 This is the first step in development beyond that simpler condition 

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

 much dilated oesophagus (Jig. 46. 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 respir- 

 ation in the Myxinoids, have a highly vascular, but not a ciliated 

 mucous surface : this is disposed in I'adiated folds, and is further in- 

 creased by secondary plicje. The seven branchial sacs on each side 

 of the oesophagus have short external ducts (Jig. 66. k), Avhich open 

 by as many distinct orifices in the skin, in a species of Bdellostoma 

 hence called heptatrema (prep. 1018.) : the internal branchial ducts 

 communicate by as many openings (ib.y) 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 internal duct {Jig. 61. f) communicates 

 wath a median canal, beneath and distinct from the oesophagus, ter- 

 minating 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. 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 Lo- 

 phioid fishes ; in some of the small Frog fishes (Antefinm-ius), the 

 circular branchial pore is produced into a short tube above each pec- 

 toral fin. The power of existing long out of water depends chiefly 

 on these mechanical modifications for detaining a quantity of that 

 element in the branchial sacs ; for fishes perish Avhen taken out 

 of water, chiefly by the cohesion and desiccation of their fine vascular 

 branchial processes, through which the blood is thereby prevented 

 from passing.* If sufliicient water can be retained to keep the gill- 

 plates floating, the oxygen which is consumed by the capillary bran- 

 chial circulation is supplied to the water retained in the branchial 

 sac directly from the air. In some of the Eel tribe the small branchial 

 outlets are closely approximated below, as in Sphagebranchus ; and 



* cvi. p. \2A. 



