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AQUATIC VERTEBRATES 



number of distinct advances over the lamprey. It has a mouth with 

 movable jaws lined with several rows of sharp teeth. It has a pair of 

 nostrils at the front of the head. These are used for the sense of smell 

 only and play no part in respiration. Sharks have a very keen sense 

 of smell and become very excited when they detect the scent of blood 

 in the water. There are five gill pouches on each side of the head and 

 these open to the outside by means of five gill slits. Water is taken in 

 through a pair of spiracles on top of the head, passes over the gills 

 which line the gill pouches, and goes out through the gill slits. Em- 

 bryological studies show that the spiracle is homologous to the first gill 

 slit in the lamprey. It is lined with vestiges of gills, but these do not 

 function in respiration. 



Photo by Winchester 



Fig. 25.2. Tiger shark. This is a splendid specimen of the tiger shark photographed 

 at Marineland aquarium in Florida. Note the small shark sucker attached to the mid- 

 ventral surface of the shark. The shark sucker is not a parasite but merely goes along 

 for the ride and sometimes shares in the food eaten by its huge companion. This is 



an illustration of commensalism. 



A shark bears two paired appendages, one in the shoulder region, 

 the pectoral fins, and one in the hip region, the pelvic fins. Internally 

 these are attached to the pectoral girdle and the pelvic girdle, respec- 

 tively. There is a segmented vertebral column composed of cartilage 

 and vestiges of the notochord. The brain is completely enclosed in a 

 cartilaginous skull. 



We ordinarily think of ears as organs of hearing, but the shark has 

 ears that do not function in this sense. Balance is the primary sense 

 which is localized in the ears, and the secondary function of hearing 

 does not appear until the land animals have developed. The shark has 

 a pair of internal ears embedded in the cartilage of the skull. Each 

 ear consists of three pairs of semi-circular canals and a central chamber. 

 If these ears are removed, the shark cannot tell whether it is right 



