THE ORIGIN OF THE ANIMAL KINGDOM. 7 



79 



changes, however, need not delay us. They are 

 technical in nature and could only be understood by 

 one acquainted with comparative anatomy and em- 



FIG. 8. Diagrams illustrating the origin of the Coelentera Mollusca, and 

 Annelida from the gastrula. To produce the Ccelentera a becomes directly 

 modified into^-. To produce the Annelida the line of development is, ,<:, d. 

 To produce the Mollusca it is a, b, e^f. In/" the twist that appears in the shell 

 twists the digestive canal, m, mouth ; , anus ; j/z, shell ; wy, cells which are 

 to develop into muscles. 



bryology. Nor, indeed, must it be understood that 

 the embryologist can follow them in all cases even to 

 his own satisfaction, nor that all embryologists agree. 

 Various methods of accounting for the origin of the 



UJI7H.RSITT; 



-W 



