AQUATIC RESPIRATION OF FISHES 9 



some teleost larvae (e.g., Gymnarchus), are prolongations of the 

 gill filaments of the posterior hemibranchs. Their function in 

 nutrition is probably as important as in respiration. The external 

 gills of the free-living larvae of lungfish, Polypterus (a primitive 

 bony fish), and Amphibia develop as outgrowths from the outer 

 dorsal surface of the gill bars. They are therefore ectodermal in 

 origin. 



(a) RESPIRATION IN CYCLOSTOMES 



One diagnostic character of lampreys and hagfishes is the 

 presence of a relatively large number of gill slits. In the larval 

 lampreys (Ammocoete) the respiratory current is produced by a 

 velum which is also important in feeding. Food is trapped on the 

 slits as in Amphioxus by means of mucus secreted by an endo- 

 style.* These slits communicate between the pharyngeal cavity 

 and the outside, but in adult lampreys the internal openings are. 

 to a separate diverticulum of the oesophagus. In the hagfishes, 

 Myxine and the Bdellostoma, the gills open directly into the 

 pharynx. The slits open separately to the outside in lampreys but 

 in Myxine the external openings have coalesced so that only a 

 single opening is visible on the outside of the animal. In addition 

 on the left side there is a duct which connects the oesophagus 

 with the exterior. This oesophageo-cutaneous duct is homologous 

 with a gill slit but any gill lamellae are absent. In other hagfishes 

 (e.g. Paramyxine) the coalescence of the external gill openings is 

 less complete and in forms like Bdellostoma the gill pouches 

 open separately to the outside just as in lampreys. The gill struc- 

 ture of both living and fossil cyclostomes is of the same general 

 type, usually termed marsipibranchs or pouched gills. The 

 pouches are inside the branchial skeleton which forms a com- 

 plicated elastic network. The internal position of the respiratory 

 epithelium relative to the visceral arch skeleton distinguishes 

 them from the gills of true fishes, as does their blood supply. 

 Each afferent vessel supplies the whole of a pouch, i.e. both the 

 anterior and posterior hemibranch (fig. lb). 



* Midventral groove of the pharynx. 



