confidence in their own speed and 

 strength that they do not waste energy- 

 attacking the sharks when the sharks 

 are not aggressive. 



Some of the smaller species, in- 

 cluding the deep-water green dog shark, 

 are thought by some to hunt in packs, 

 because their stomachs often contain 

 the beaJ^s of squid and octopus, repre- 

 senting prey too large and agile for 

 these small sharks to kill and eat 

 alone. 



nervous system of the shark is de- 

 centralized to a much greater extent 

 than it is in the higher vertebrates, 

 and the areas of the brain usually re- 

 garded as thought centers are absent 

 in sharks. Sharks have been shown ex- 

 perimentally to be capable of develop- 

 ing rather complicated responses to 

 conditioning stimuli and may thus 

 be able to learn at the reflex 

 level. 



Anatomical Features 



Sensory Perception 



Studies on sensory perception of 

 sharks have been difficult to carry 

 out satisfactorily. In the broadest con- 

 notation, the senses of sharks are 

 similar to those of other animals. The 

 sense of smell is presumed tobe highly 

 developed and has been shown experi- 

 mentally to play a major role in food 

 finding. Sharks also have a sense 

 corresponding to our sense of taste, 

 but organs for tasting are widely 

 scattered over the skin of the head and 

 even other parts of the body. Although 

 this sense is in some way related to 

 feeding habits, it is possible that in 

 sharks it serves other functions as well. 

 Sharks' eyesight is apparently adapted 

 best for the detection of movement, but 

 their eyes are extremely complicated 

 structures, adapting especially to low 

 light levels. 



Some sharks have calcareous 

 bodies in the chamber of the inner ear 

 which are presumed to be associated 

 with the sense of balance. Sharks are 

 responsive to noises and pressure 

 w^aves, but the importance of sound 

 and pressure differentials to sharks is 

 not well understood. Experimental pro- 

 cedures to determine the roles of the 

 ears (internal) and the lateral line 

 organs have met with only limited 

 success. 



Sharks apparently have a very low 

 intelligence level and many, if not all, 

 of their activities are reflexive. The 



In addition to the general anatomi- 

 cal features described in the introduc- 

 tion, each species of shark has its own 

 structural adaptations. Many of these 

 features can be correlated with a more 

 or less specific way of life assumed by 

 the shark species. A few examples of 

 divergence from the generalized 

 shark pattern and of adaptive fea- 

 tures can be given here, many more 

 exist. 



The angel sharks, found on both 

 coasts of the United States, diverge 

 from the typical fusiform shape and 

 display a flattened appearance char- 

 acteristic of rays. A second raylike 

 shark, although not flattened, is the 

 sawshark of the deep waters of the 

 Straits of Florida which resennbles 

 closely the sawfish of shallow coastal 

 waters. Sawfish, though sharklike, are 

 true rays. In spite of the superficial 

 resemblance to rays exhibited by 

 these two shark species, close ex- 

 amination shows that their more 

 basic characteristics are those of 

 sharks. 



The thresher shark has a pecul- 

 iarly enlarged and elongated upper 

 caudal lobe which, according to fisher- 

 men observers, it uses to stun its prey 

 by "threshing" around in a school of 

 fish. Its tail is one of the toughest 

 flexible structures developed in the 

 animal kingdom. Many other correla- 

 tions between anatomy and mode of life 

 can be pointed out. Fast- swimming, 

 oceanic sharks, such as the mako and 



