RESPIRATORY ORGANS. 



2 39 



from the bronchus by the development of a short afferent duct, while the demi- 

 branchs come to lie in oval pouches (much as in Myxine, fig. 245), in allusion to 

 which the cyclostomes are sometimes called marsipobranchs (pouched gills). 



In the myxinoids the tract between the mouth opening and the pharynx is 

 more elongated and the pharyngeal region (fig. 244, A) is not differentiated into 

 oesophagus and bronchus, as in the adult lampreys. In Myxine there are six pairs 

 of gills; in Bdellostoma the number ranges from seven to fourteen, varying even on 

 the two sides of our Pacific species, B. dombeyi. In the petromyzons and in 



A 



FIG. 246. Diagram of gill clefts in (.4) elasmobranchs and (B) teleosts. A' and B', 

 a single gill of each, a, artery; br, branchial ray; d, demibranchs ; gc, gill chamber; gr, 

 gill raker; o, operculum; oe, oesophagus; oo, opercular opening; s, spiracle; v, veins. 



Bdellostoma the efferent ducts of the gill pouches open separately to the exterior; in 

 Myxine (fig. 245) they unite into a common duct on either side, the left also receiv- 

 ing an cesophago-cutaneous duct, behind the last gill. This duct, which leads 

 from the oesophagus to the exterior, resembles a gill cleft, but lacks gills. A similar 

 duct occurs in the same position in Bdellostoma. 



In the fishes there are two types of gills and associated structures. 

 In the elasmobranchs (the chimaeroids excepted) the interbranchial 

 septum is greatly developed (fig. 246, A'}, extending some distance 

 laterally beyond the gill folds so that the distal part of the cleft forms an 

 excurrent canal. This prolongation of the septum extends to the ex- 

 terior and then turns backward, thus protecting the delicate gills from 



