THE ELASMOBRANCH FISHES 



149 



tends from the visceral skeleton outward and is attached to the inte<4'unient; 

 it is further secured in its outer margin by the dorsal and ventral extra- 

 branchial cartilages, where such exist. These extrabranchials (see p. 45, fig. 49, 

 ex.b.) extend around the margin of the septum roughly at right angles to the 

 branchial rays. At their dorsal and most ventral angles the extrabranchials 

 pass posteriorly across the pockets so that 

 in the opened pocket their transected ends 

 show on both sides of the incision (ex.h., fig. 

 142). 



Gill Filaments 



csd. 



Fig. 143. Section 

 cutting parallel 

 to branchial fil- 

 nments tlirougli 

 second holo- 

 branch, Heptan- 

 chus maculatus. 

 (H. M. Gilkey, 

 del.) 



m 



The respiratory membrane is formed of 

 the series of folds or filaments {fl.) attached 

 to the anterior and posterior sides of the sep- 

 tum of each whole gill. Anteriorly these folds 

 compose the respiratory surface of the an- 

 terior demibranch; posteriorly, that of the 

 posterior demibranch. The filaments of the 

 posterior demibranch in Hepfanchus macu- 

 latus extend considerably farther distally on 

 the septum than do those of the anterior set, 

 but they also arise farther out from the base. 

 In both they consist of a series of plates flat- 

 tened from side to side. The longest of these 

 plates is located medially and from this point 

 they get shorter and shorter until at both the 

 dorsal and ventral angles of the pocket they 

 are relatively minute. Above and below the 

 epi- and ceratobranchial segments filaments 

 are absent. 



A section taken at right angles to the in- 

 ternal branchial arch and cutting through 

 the epibranchial segment, parallel with an 



anterior and a posterior filament, is seen in figure 143. In this, the fourth dor- 

 sal constrictor muscle (csd.) is thickened toward the margin of the septum, 

 where it is supported by the extrabranchial cartilage (ex.b.). It is continued 

 toward the internal branchial aperture as an interbranchial muscle (ib.d.) 

 directly in front of the cartilaginous branchial ray (b.r.), the upper part of 

 which in figure 143 has been cut off. Anterior to the branchial ray is the third 

 afferent artery (af.) which is surrounded by the nutrient vein (sinus) of the 

 arch, and anterior to the union of the branchial ray with the internal branchial 

 arch is the large anterior efferent-collector artery (efc.^) ; just posterior to 

 this efferent-collector is the pretrematic division of the branchial nerve (cross- 

 hatched), and below the posterior filament is the small posterior efferent- 

 collector artery (efc.^). 



ad.-^ 



ad., adductor 

 muscle; a/., third 

 afferent artery ; 

 b.r., branchial 

 ray cut short; 

 csd., fourth dor- 

 sal constrictor 

 muscle; eb., epi- 

 branchial seg- 

 ment of internal 

 branchial arch; 

 efc.*''% fourth 

 and fifth effer- 

 ent-collector ar- 

 teries; ex.b., ex- 

 trabranchial car- 

 tilage; fl.a., an- 

 terior filament ; 

 fl.p., posterior 

 filament; ib.d., 

 dorsal inter- 

 branchial mus- 

 cle ; 11., posterior 

 division of the 

 branchial nerve. 



