HYDROIDS. 49 



HYDROIDS. 



UNDER this order, the general character of which has already 

 been explained in the introductory chapter on Acalephs, are in- 

 cluded a number of groups which, whether as Hydroid commu- 

 nities in their earlier phases of existence, or as free swimming 

 Medusas in their farther development, challenge our admiration, 

 both for their beauty of form and color, and their grace of motion. 

 Some of them are so minute that they escape the observation of 

 all but those who are laboriously seeking for the hidden treas- 

 ures of the microscopic world, but the greater number are large 

 enough to be readily found by the most inexperienced collector, 

 when his attention is once drawn to them ; and he may easily 

 stock his aquarium with these pretty little communities, and 

 even trace the development of the Jelly-fishes upon them. 



To the Hydroids belong the Campanularians, the Sertularians, 

 and the Tubularians. Some examples of each, as represented on 

 our shores, will be found under their different heads, accompa- 

 nied with full descriptions. There is another group usually con- 

 sidered as distinct from Hydroids, and known as a separate order 

 among Acalephs, under the name of Siphonophorae, but included 

 with them here in accordance with the views of Vogt, Agassiz, 

 and others, in whose opinion they differ from the ordinary Hy- 

 droid communities only in being free and floating, instead of 

 fixed to the ground. Some new facts, published here for the 

 first time, tend to sustain the accuracy of this classification.* 

 With these few preliminary remarks to show the connection of 

 the order, let us now look at some of the animals belonging to it 

 more in detail. 



Campanularians. 



All the Campanularians, of which Oceania (Fig. 68), Clytia 

 (Fig. 73), and Eucope (Fig. 61) form a part, belong among 

 those little shrub-like communities of animals called Hydroids, 



* See Chapter on Nanomia. 



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