i66 



AN INTRODUCTION TO ZOOLOGY 



does not arise in this way, for it is formed directly by the modification 

 of a hollow bud from the blastostyle, and is never at any time a 

 polyp. The above stages, however, are not purely imaginary, for 

 we have indications that the gastral cavity is at first extensive, and 



that the entoderm lamella is formed in the way suggested. It 

 serves to show how nearly alike the two forms are, and perhaps we 

 are justified in regarding them as modifications of the one type each 

 adapted for its own particular mode of life. The medusa is a free- 

 swimming wanderer, and needs therefore to have specially developed 

 muscular and sensory systems, which in turn necessitate an increase 



