GILLS OR BRANCHIAE 



255 



to the adult condition the oesophagus grows forward, dorsal to the gill clefts, as 

 far as the anterior end of the pharynx, so that there is cut oflf a ventral respira- 

 tory tube, the branchial duct or 'bronchus' (fig. 272, B). At the same time 

 he gill-bearing part of each cleft becomes separated from the branchial duct 

 by a short afferent duct, while the part of the cleft containing the gills is trans- 

 formed into an oval pouch (much as in Myxine, fig. 273), in allusion to which the 

 cydostomes are often called marsipobranchs (pouched gills). 



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

 ind the pharyngeal region is never differentiated into oesophagus and branchial 

 duct (fig. 272, .4). Myxine has six pairs of gills; Bdellostoma has from seven 



Fig. 274. — Diagram of gill clefts in {A) elasmobranchs and (5) teleosts. A' and 

 B', a single gill of each, a, artery; ba, branchial arch; br, branchial ray; d, demibranchs; 

 gc, atrial chamber; gr, gill raker; 0, operculum; oe, oesophagus; 00, opercular opening; 

 *, spiracle, in A', septum; v, veins. 



to fourteen, the number varying even on the two sides of our Pacific coast species, 

 B. dombeyi. In Bdellostoma and the lampreys the efferent ducts of the gill 

 pouches open separately to the exterior; in Myxine (fig. 273) they unite to a 

 common duct on either side, with a single external opening. In both Bdello- 

 stoma and Myxine an oesophageo-cutaneous duct leads from the left side of the 

 oesophagus, either into the common efferent duct or directly to the exterior. 

 It resembles a gill cleft in all except the unpaired condition and the absence of 

 gills. 



Fishes have two types of- gills and associated structures (fig. 274). 

 With the exception of the chima^roids, the elasmobranchs have the 

 interbranchial septum greatly developed (fig. 274, i4), so that it ex- 



