INTRODUCTION. 



XXX IX 



cases we have the fixed and the free condition of the gono- 

 zooid within the limits of a species, the former being 

 abnormal and the result of partial development, but 

 representing a perfect and permanent form in another 

 portion of the series. 



It only remains to give a short account of the develop- 

 ment of the Hydroid embryo. (Woodcut, fig. xvi.) The 



Fig. xvi. 



a- 



1. The planula of a Carupanularian Hydroid. 



2. The same, in a more advanced stage. a. The enlarged extremity, by 

 which the embryo attaches itself, b. The chitinous film. c. The point at 

 which the polypite is developed. 



3. The young Campanularian soon after attachment. a. One of the tubular 

 lobes into which the disk divides. 



The arrows show the direction in which the planulc moves. 



ovum after impregnation passes through the various stages 

 of segmentation, and is resolved at last into a minutely 



