METAZOA COELENTERA. 89 



COELENTERA. 

 Section 2. HYDROZOA. 



HYDROPHORA. 



If we consider the Protozoa and Mesozoa as constituting 

 the trunk of our genealogical tree and the Porifera as the 

 first short branch sent off from this trunk, then the Coelen- 

 tera through comparative simplicity of structure represent 

 the second branch. Although an unbroken line of descent 

 from the many-celled, one layered Mesozoan to the Hydro- 

 zoa (the most generalized class of the Coelentera) cannot 

 be traced, yet it is not difficult to conceive of an animal 

 like a primitive Hydroid arising from an ancestral form 

 similar to that which produced in course of generations 

 the simplest, tubular sponges. 



The two theories held by naturalists in regard to the 

 origin of the Metazoa have already been stated (see p. 

 64). Briefly summarized it may be said that, according 

 to one view, the one layered blastula gives rise to a two 

 layered invaginate gastrula, the ancestral form of which, 

 the Gastraea, has not been discovered. The gastrula in 

 turn produces a form that is two layered in youth and 

 three layered in the adult, like the sponge. 



According to the second theory, the blastula gives rise 

 to a solid parenchymella which in time becomes two lay- 

 ered and hollow and afterward is provided with an open- 

 ing. In this case no primitive invaginate gastrula exists. 

 The Hydrophora or Hydromedusae, now to be described, 

 illustrate almost universally the second mrde of develop- 

 ment, and some naturalists l even maintain that not a single 

 invaginate hydroid gastrula has been observed. 



1 W. K. Brooks, Mem. Boston Soc. Nat. Hist., Ill, no. 12, 1886, 

 p. 401. 



