v. 4 



SKIN OF ELASMOBRANCHS 



141 



Expansion of the front end is developed at first to compensate the 

 effect of the tail, but the pectoral fin becomes expanded to allow ver- 

 tical adjustments and then reduction of the hypocaudal lobe of the tail 

 accompanies the adoption of life on the sea bottom. Eventually all 



Fig. 95. Development of denticles in the dogfish. 



A and b, first gathering of odontoblasts (sc.) below the basement membrane (brn.); ml. are the 



epidermal cells that will become modified, c, first deposition of dentine (d.). In D there is 



more dentine and a pulp cavity (p.) is seen. In E are shown stages in the formation of enamel 



(e.) and of the basal plate (bp.) while the denticle cuts the epidermis (ep.). 



(From Goodrich, Vertebrata, A. & C. Black, Ltd.) 



locomotion is produced by undulatory movements of the fins, which 

 were at first used only to raise the fish off the bottom. 



4. Skin of elasmobranchs 



Being swift and predatory animals, more attackers than attacked, 

 the sharks do not possess a very heavy external armament. The skin 

 itself is tough, being covered by layers of epidermis. Beneath this is a 

 thick dermis of connective tissue with fibres arranged at right angles 



