MOUTHS AND JAWS 105 



(Fig. 45B), but in the primitive Frilled Shark [Chlamydoselachus) 

 the wide mouth occupies a completely terminal position (Fig. 

 32c). In other Sharks the position of the mouth seems to have 

 been brought about by the forward prolongation of the front 

 part of the head above the jaws to form a snout or rostrum, a 

 modification clearly designed to gain increased speed. From 

 the position of the mouth it is generally assumed that a Shark 

 is obliged to turn over on to its side or back in order to engulf 

 its food. This, however, is by no means always so, and, although 

 it may turn over when taking food at the surface, as in the case 

 of a lump of meat thrown overboard from a ship, it has fre- 

 quently been observed to maintain its normal position when 

 seizing living prey, and to push the snout out of the water in 

 order to bring the jaws into play. 



In the sluggish, ground-Hving Rays {Hypotremata) the mouth 

 is nearly always well under the head, and generally takes the 

 form of a straight sHt (Figs. 14B; 107). Although for the 

 most part a rather inactive creature, a Ray (Raia) will display 

 great activity when a small fish or crustacean comes within its 

 reach. Owing to the position of the mouth it is unable at once 

 to seize its prey, but darts rapidly over it, covers it with its 

 body and enlarged pectoral fins, and devours it at leisure. The 

 large Rays known as Sea Devils [Mobulidae) differ from the 

 remainder in living more or less in the open sea, and the 

 mouth, in some of them at least, lies nearly at the end of the 

 head. These fishes are remarkable in having the front parts 

 of the pectoral fins prolonged forward to form a pair of fleshy 

 appendages having the appearance of horns (Fig. 13A). They 

 are known as the cephalic fins, and the part played by these 

 structures in obtaining food diflfers somewhat in the various 

 species. The Smaller Devil-fish {Mobula) is in the habit of 

 swimming about in shallow water near the shore in shoals of 

 three to five in number. The "horns" are normally kept 

 tightly curled up to a sharp point, but no sooner is a shoal of 

 small fishes sighted than the Rays swing round in a semi- 

 circle, and rush their intended meal to the beach; at that 

 instant the cephalic fins flash open, and meeting below the 

 mouth form a funnel through which the flshes are conveyed to 

 their jaws. The great Sea Devil {Manta), on the other hand, 

 is a more or less solitary feeder, and swims along slowly near 

 the surface, turning from one side to the other with the cephalic 

 fins in constant movement, and actually using these appendages 

 to throw the food into the mouth. 



