VI 



RESPIRATORY TRACT 



RESPIRATORY TRACT OP HEPTANCHUS MACULATUS 



HeptancJuis is characterized by having the greatest number of gill clefts or 

 external branchial apertures of any kno\vii Elasmobranch. In fact it is from 

 the number of clefts, as we have said, that it has received its name. These per- 

 forations in Heptanchns are lateral in position and lie between the cranium 

 and the pectoral fin. The first branchial cleft, not the spiracle, is the largest of 

 the series, while the one farthest from the cranium is the smallest. By opening 

 one of the clefts both dorsally and ventrally we may study the structures of 

 the gill pouch or pocket. 



Gill Pouch or Pocket 



If the second gill pocket of Heptanchus (fig. 142, facing p. 148) be opened 

 as suggested above, the cavity is seen to be shaped somewhat like a funnel 

 flattened from side to side, with the apex pointing inward and opening into 

 the pharynx as the internal branchial aperture. The internal branchial aper- 

 ture of the second pocket is supported by the first branchial arch anteriorly 

 and by the second branchial arch posteriorly. On the margin of the first arch, 

 and pointing backward, are one or two low projections which resemble in- 

 cipient gill rakers. On the anterior branchial arch are also numerous eleva- 

 tions, the bases of the cartilaginous branchial rays (b.r.), which support the 

 gill septum. The tips of these cartilages are visible on the posterior surfaces. 

 The walls of the pocket, both anteriorly and posteriorly, are covered with nu- 

 merous radiating gill filaments or folds (fl. ) , the effective organs of respiration. 

 The remaining pockets, excepting the seventh, are essentially similar to the 

 first. The seventh external branchial cleft has the same funnel-shaped arrange- 

 ment with its smaller apex connecting with the pharynx. On its anterior wall 

 filaments similarly appear although these are fewer in number than in the 

 preceding pockets. The posterior wall, however, is perfectly smooth, all sem- 

 blance of gill filaments being entirely wanting. 



Spiracle 



The spiracular pocket upon being opened is found to be similar to the last 

 pocket, that is, it bears filaments only on the anterior wall. But it differs from 

 the others in the reduction in size of both its external and internal apertures 

 and in the reduction in the number of its filaments. 



The spiracle in Heptanchus is not a straight tube, but as Ridewood (1896) 

 says, it bears diverticula on its walls. The most important of these diverticula 



[147] 



