THE LOWER FISHES 139 



desirable for another reason. Sharks are heavier than water and possess no 

 swim bladder as do the bony fishes. They must swim slowly but constantly 

 or they will sink. Some species have become so attuned to constant motion 

 that they use it to help circulate water through the gills and will suffocate 

 if held stationary. This constant motion of the active species makes them 

 appear menacing. 



The fastest sharks are those in which the bodv is shortened and compact and 

 the tail lunate, that is, has nearly equal lobes. These are the mackerel sharks, 

 family Isuridae. One shark familv, the hammerheads or Sphyrnidae, has made 

 an attempt to increase maneuverability by extreme flattening and widening of 

 the head which is then used as a forward rudder. Another attempt in increased 

 maneuverability resulted in the flattened rays, where the pectoral fin has been 

 greatly expanded and has gradually taken over all locomotion. 



All sharks and ravs have internal fertilization, which is uncommon in bonv 

 fishes. The males possess claspers on the pelvic fins which are inserted into the 

 females. A few sharks lay eggs and are said to be oviparous. Most are ovovivip- 

 arous, the eggs hatching in the female's body and the young being born alive 

 shortly thereafter. A few sharks are truly viviparous as are mammals and have 

 a "placenta" by means of which the females nourish the pups as they grow. Some 

 ovoviviparous rays have a sort of internal feeding in which the female's 

 "uterus" secretes a milkv substance for the young. Sharks and ravs produce 

 few young, the recorded extremes in numbers varying from one to eighty-two, 

 and perhaps averaging below twenty. 



All sharks and rays are carnivorous. A great variety of food is eaten, but the 

 sharks are mainly fish-eaters and the rays mainly mollusc- and crustacean-eaters. 

 A few are cannibalistic. Tiger sharks are especiallv liable to eat other sharks 

 and rays when these are disabled or hooked. Hammerheads like to eat rays. 

 Gudger (1932) reports finding fifty-four spines of the eagle ray, Aetohates 

 narinari, and sting ray, Dasyatis say, in the head and jaws of one large 

 hammerhead. There is a record of the skate, Raja, eating other small rays. 



The Sharks: Order Pleurotremata {"side openings") 



The typical shark is commonly visualized as a large, ferocious, swift predator 

 with very little on its single-track mind other than the general destruction of all 

 life of the sea, including man. We shall soon see how incorrect this vision is, 

 for while it is true that some sharks are among the largest and most voracious of 

 animals, most sharks do not conform to such a pattern. 



In general, the sharks are fairly stout and subcylindrical of body, but several 

 are almost as flattened as rays and even parallel the rays in habits. Others are 

 long and slim. They range in size from 18 inches to nearly 60 feet. Several 

 dwell in the deep sea, and some pelagic species are brightlv luminescent. 



Even though we must lay aside the stereotyped idea of what a shark looks 

 like, there are three characteristics that all sharks possess: (1) five to seven gill 

 slits on the sides just back of the head (hence the name "Pleurotremata"), (2) 

 pectoral fins that may be large or small but are never connected to the head, 

 and (3) definite eyehds. In all of these traits they diff^er distinctly from rays. 



