10 THE SKATE 



B. Homology and Analogy. 



The structure and mode of origin of placoid scales and 

 the teeth of all vertebrates are exactly the same, although 

 their function and superficial appearance is very different. 

 Teeth and placoid scales then are said to be homologous 

 structures. Analogous structures are those structures 

 which resemble each other in superficial appearance or in 

 function but which have different origins. The lining of 

 the mouth cavity is composed of skin which has become 

 turned in, consequently the placoid scales which were car- 

 ried with it into the mouth cavity have been adapted to 

 new purposes and the homologous parts have begun a 

 divergence. Analogous structures result from a con- 

 vergence which is often the result of the action of similar 

 conditions upon structures of different origin. Identify 

 the following parts on the longitudinal section of a verte- 

 brate tooth; the crown, homologous with the spine; the 

 root homologous for the most part with the basal plate; 

 the pulp cavity, a central space, filled in the living tooth 

 with the dermal papilla of connective tissue, blood vessels, 

 nerves, etc.; the dentine, a bone-like substance composing 

 most of the tooth and the enamel the shiny outer coating 

 of dentine on the crown. Compare part for part the placoid 

 scale and the vertebrate tooth. 



The theory of evolution accounts for homologous struc- 

 tures by its central concept that all vertebrates have been 

 derived from a common ancestor. Consequently homolo- 

 gous structures would be the result of the inheritance of 

 modifications of a common primitive structure. 



Drawing 5. Draw the skate from the ventral surface. 

 Drawing 6. Draw a section of a placoid scale and beside it 

 diagram the tooth of a vertebrate. 



