354 



COMPARATIVE ANATOMY 



backwards from the mouth, a ventral branchial or respiratory 

 and dorsal oesophagus (Fig. 259, B). Inspiration as well as expira- 



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B 



FIG. 259. DIAGRAM OF A LONGITUDINAL SECTION THROUGH THE HEAD OF 

 THE LARVAL (A) AND ADULT (B) LAMPREY. 



tion takes place through the gill-apertures when the animal is 

 attached by its suctorial mouth. 



In Petromyzon and Bdellostoma l the individual branchial sacs, 

 which communicate directly with the pharynx, open freely to the 

 exterior : in Myxine this original condition becomes modified by 

 the outer parts of the gill-passages growing out into long tubes, 



JnfHHML 



Cli A' 



FIG. 260. LONGITUDINAL SECTION THROUGH THE HEAD OF A LARVAL LAMPREY. 



l>, c, ventricles of the mid- and hind-brain ; Ch, notochord ; Ep, epiphysis ; 

 HH, ML^ hind-brain ; Juf, infundibulum ; K, K, K, the three anterior gills ; 

 N, nasal sac ; o, subdural cavity ; P, papillae of imicous membrane ; R, spinal 

 cord ; Th, thyroid (hypobranchial furrow) ; V, velum ; *, communication 

 between the ventricle of the olfactory lobe and that of the telencephalou, 



which unite to form a common duct on either side ; this opens far 

 behind the branchial apparatus on the ventral side of the body. 



1 In Bdellostoma there are usually six or seven pairs of branchial sacs, and 

 behind these, on the left side, an <nxopha.geo-cutane.ouis duct opens directly into the 

 pharynx, as is also the case in Myxine. Bdellostoma bischoffi and B. stouti 

 possess eleven or twelve, and B. polytrema thirteen or fourteen pairs of gill- 

 pouches. 



