RESPIRATORY ORGANS. 239 
from the bronchus by the development of a short afferent duct, while the demi- 
branchs come to lie in oval pouches (much as in Mywxine, 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 
cesophagus and bronchus, as in the adult lampreys. In Mywine 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 
Fic. 246.—Diagram of gill clefts in (A) 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; 0, operculum; ve, cesophagus; 00, opercular opening; s, spiracle; v, veins. 
Bdellostoma the efferent ducts of the gill pouches open separately to the exterior; in 
Mysine (fig. 245) they unite into a common duct on either side, the left also receiv- 
ing an oesophago-cutaneous duct, behind the last gill. This duct, which leads 
from the cesophagus 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 chimeroids 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 
