CHAPTER XXVI 



ELASMOBRANCHII* 



Unlike the cyclostomes, the Elasmobranchs are covered with scales 

 and have two sets of paired fins on the ventrolateral surfaces of the 

 body. In addition to these, there are unpaired or median fins. The 

 gill apertures, except for the first, or spiracle, are slitlike instead of 

 circular, as seen in the lamprey. The gills are supported by gill 

 arches, and the mouth has an upper and lower jaw. The skeleton is 

 entirely cartilaginous, there is a partially persistent notochord, and 

 the exterior is covered and protected by placoid scales. The males 

 have a modification of each pelvic fin known as a clasper which is 

 used as a copulatory organ. 



The mouth is not right at the anterior end of the body but is ven- 

 tral or subterminal. There is present in the ileum of the small in- 

 testine a spiral valve which increases the internal surface, thus add- 

 ing absorptive area. The Elasmobranchs have no operculum or air 

 bladder. 



Classification 



The class is divided into two rather easily distinguished sub- 

 classes. The first group is very common to American shores and 

 the second is rarely seen in our waters. 



Subclass Selachii. — This group includes the sharks which are 

 cylindrical in shape, possess laterally located gill slits, and are active 

 swimmers; and the rays, which are dorsoventrally flattened, possess 

 ventrally located gills, are less active, and dwell on the bottom of 

 the sea. This subclass is usually divided into two orders. 



Orders Euselachii and Cyclospondyli. — The sharks make up these 

 orders. The dogfish sharks (Squalus acanthias and others), tiger 

 shark (Galeocerdo arcticus [Faber]), cub shark {Carcharias platydon 

 [Poey]), shovelhead or bonnethead shark {Reniceps tiburo [Linn], 

 and man-eater shark {Carcharodon carcharias [Linn]) are forms com- 

 monly found. The majority of sharks are carnivorous and active, 

 but they rarely attack man unless the person is already wounded. 



•In collaboration with Miss Mary Fickling. 



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