214 THE DOG-FISH. 



D. The Scales and Teeth. 



1. The scales. The whole surface of the body is covered 



with small placoid scales, which are special 

 developments of the skin. 



a. General arrangement. The scales are larger on the 



dorsal than on the ventral surface, and differ 

 somewhat in shape in different parts of the 

 body. Their projecting points are directed back- 

 wards, so that the finger can be readily passed 

 along the body from the head to the tail, but 

 meets with considerable resistance in the reverse 

 direction. 



b. Structure of the scales. 



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

 in a solution of caustic potash to isolate the scales. Mount 

 them in glycerine, 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 

 bet<veen the exceedingly simple condition seen in Amphioxus 

 and the complicated bony framework of the higher verte- 

 brates. 



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 quite reach 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 



