198 THE DOG-FISH 



b, Structure of the scales. 



Cut out a small piece of skin, and boil it for a few minutes 

 in caustic potash to isolate the scales. Mount them in glyce- 

 rine, and examine with the microscope. 



Each scale consists of a bony base, usually 

 four-lobed, from which a pointed spine of dentine, 

 capped with enamel, projects backwards. 

 2. The teeth are really the enlarged scales of the skin 

 covering the jaws. They are arranged in several 

 rows, and have their points directed backwards. 



II. THE SKELETON. 



The skeleton of the dog-fish forms an important link 

 between the exceedingly simple condition seen in Amphioxus 

 and the complicated bony framework of the higher vertebrates. 



In the embryo a rod-like notochord extends down the 

 back, immediately beneath the central nervous system, and 

 dorsal to the alimentary canal, similar to that of Amphioxus, 

 except that it does not reach quite to the anterior end of the 

 body. Around this notochord a sheath of cartilage is formed, 

 which in the head gives rise to the base of the skull, and in 

 the body becomes divided transversely into segments the 

 centra or bodies of the vertebras, the flexibility of this region 

 being thereby increased. 



Above the centra neural arches are formed, which sur- 

 round and protect the brain and spinal cord ; and below them 

 haemal arches, which tend to enclose the body-cavity. 



Besides this axial skeleton a visceral skeleton is developed 

 around the anterior part of the alimentary canal, and skeletal 

 elements are also formed in relation with the fins. 



The skeleton consists throughout life of cartilage. It is 

 hardened in many places by deposition of calcareous salts, 

 but no true bone is developed in it. 



The skeleton of the dog-fish is best prepared by taking 

 a fresh specimen, dipping it in hot water a few degrees below 



