THE ELASMOBRAN(!II FISHES 



37 



A transverse section tliroiig'h the sting of Vrolophus (p. 29, fig. 32) sliows 



tliat it is convex dorsally. The ventral side consists of right and left plane 



surfaces separated by a median ventral ridge. Both the plane surface and the 



convex dorsal sides are covered with a thin layer of enamel (e.) under which 



is the thick dentine {d.). But the dentine here is arranged differ- 



A ently from that of the placoid scale. 



I I Figure 43 is drawn from the dorsal side of a sting and represents 



it as a transparent object. In it may be observed numerous longi- 

 tudinal and anastomosing canals (r.) surrounded by heavy dentine 

 (f/.). This arrangement of canals is much like that which we have 

 observed in the saw tooth of Prist is. 



The sting is clearly a protective structure. In those rays in which 

 it is attached nearer the body, as in Myliobatis, it is more effective 

 than in types like Dasyafis, in which it is located farther out on the 

 tail. In either type, the sting is brought into action by thrusting the 

 tail upward and forward over the back. In the thrust the weapon is 

 driven forward with precision and force, and is removed with great 

 difficulty from the wound, the recurved hooks (sp.) forming a 

 most painful tearing surface unless the sting can be pushed en- 

 tirely through. 



Gill Rakers 



lii 



Fig. 42. 

 Sting of 

 sting rav. 



In some forms, structures 

 internal branchial arches 

 rakers, such, for ex- 

 ample, as are seen in 

 Squalus sucklii (p. 154, 

 fig. 147, gr.). These gill 

 rakers evidently serve 

 as .strainers to prevent food from 

 passing out with the respiratory 

 current. Structures somewhat like 

 these in function but very unlike 

 them in form have become remark- 

 ably specialized in Cf^or/rnnrs »(fl,r('- 

 nius (fig. 44 ) and in Rhinodon typi- 

 cus, in which they form a highly 

 complex straining apparatus. Each 

 raker in Ceiovliinus arises from a 

 semilunar base and extends as a 

 long slender filament across the in- 

 ternal branchial aperture (figs. 44 

 and 148). 



It has long been known that one of 

 structurally like a placoid scale. In a 



located on the pharyngeal walls of the 

 have undergone modification into gill 



Fig. 43. Segment of sting, Vrolophus halleri, 

 seen as transparent object. Dorsal view. (A. 

 M. Paden, orig.) 



c, canals of sting; d., dentine; sp., recurved 

 spine on sting. 



these filaments with its adjoined base is 

 section through the base numerous den- 



