CHAPTER \TI 



HYDROZOAX CORALS 



" When a Writer acquaints me only with his Thoughts and Con- 

 jectures, without enriching his Discourse with any real Experiment 

 or Observation, if he be mistaken in his Ratiocination, I am in 

 some Danger of erring with him, or at least am like to lose my 

 Time, without receiving any valuable Compensation for so great a 

 Loss ; but if a Writer endeavours by delivering new and real 

 Observations and Experiments to credit his Opinions, the Case is 

 much otherwise : for let his Opinions be ever so false I am not 

 obliged to believe the former, and am left at Liberty to benefit 

 mvself bv the latter ; and though he have erroneously superstructed 

 upon his Experiments, yet, the Foundation being solid, a more wary 

 Builder may be much farthered by it, in the Erection of a more 

 judicious and consistent fabrick." — Mr. Boyle, quoted by H. Baker, 

 I.e. p. 206. 



The polyps of the Hydrozoa, although presenting an ex- 

 ternal appearance very similar to that of the polyps of the 

 other Coelenterata, are much simpler in structure. 



There is normally a mouth surrounded by a crown of 

 tentacles varying in number in the different genera of the 

 group, but always filiform or digitiform in shape and without 

 lateral pinnules, but the mouth leads directly into the body 

 cavity and there is no stomodaeum and no mesenteries. 



Most of the genera of Hydrozoa form colonies by gemma- 

 tion, which are attached to the rocks or sand by root-like 

 processes, and, by various methods of ramification, give rise 

 to plant-like structures of considerable size. 



By the older naturalists they were included in that 

 strange medley of marine products called the Zoophytes. 



Before passing on to the description of the Hydrozoa 

 that form calcareous structures there are two morphological 



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