222 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 de- 

 velopments of the skin. 



a. General arrangement. The scales are larger on the 



dorsal than on the ventral surface, and form a 

 powerful caudal ' rasp ' on the sides of the tail. 

 Their projecting points are directed backwards, 

 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 glycerin, and examine with the microscope. 



Each scale consists of a calcified base, usually 

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

 tine, 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 Amphi- 

 oxus and the complicated bony framework of the higher 

 vertebrates. 



In the embryo a rod-like note-chord 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 



