ORDER II. HYDRO1DEA. 685 



ORDER II. HYDROIDEA. 



Zoophyta ventriculo tubuliformi simplicissimo ; ovulise lateribus externe 

 enascentibus. 



Zoophytes having a simple tubular visceral cavity ; ovules growing 

 externally from the sides. 



In Chapter III., the general characteristics of the Hydroidea have 

 been stated, and in the Classification, on pages 116 to 119, the prin- 

 cipal divisions in their arrangement are given. The characteristics 

 of the genera are here added, and some few species described, which 

 were obtained by the Expedition. 



FAMILY I. HYDRIDE. 



Hydroidea, gemmis maturis deciduis ; ovulis singulis, lateralibus ; ten- 

 taculis tubulatis. 



Buds deciduous when mature; ovules single, lateral; tentacles tubular. 



This family contains the single genus Hydra. Of all zoophytes, 

 these were the earliest studied ; and the famous works of Trembley 

 arid Baker have afforded perhaps more amusement and surprise to 

 their readers than any other details in natural science ever published. 

 Van Beneden has pointed out, as a distinguishing characteristic be- 

 tween them and the TubularidaB, that the tentacles are tubular, and 

 the cavity communicates directly with the internal cavity of the ani- 



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