Metazoa Obelia. 2 \ 9 



CHAPTER IX. 



METAZOA 1NVERTEBRATA. 



SECTION I. HYDROZOA OBELIA. 



IT will be remembered that in Chap. IV. sect. iii. we divided 

 plants into two great groups, Protophyta and Metaphyta. 

 Amongst animals we similarly formed the two divisions of 

 Protozoa and Metazoa. We have now completed our sur- 

 vey of the chief types of the Metaphyta, and are prepared 

 to take up the thread dropped in Chap. VI., and follow out 

 a few of the principal modifications exhibited by the various 

 groups of the Metazoa. 



As examples of the lower Metaphyta we selected Spiro- 

 gyra and Fucus. Both these forms belong to the Algae. 

 Among the lower Metazoa a similar large group exists, the 

 Hydrozoa, some of the members of which are, indeed, 

 frequently mistaken for Algae by the uninitiated. There are 

 numerous points of superficial resemblance between the two 

 groups, far apart as they really are in structure and physio- 

 logy. 



The type most frequently selected to illustrate the group 

 of the Hydrozoa is Hydra, the fresh -water 'polype,' a selection, 

 however, by no means in all respects fortunate, since its 

 method of development is far from typical. We will employ 

 it only in order to illustrate the minute structure of the 

 Hydrozoa, and describe the mode of reproduction and life- 

 history in a more generalised form, such as Obelia genicu- 

 lata. 



