HYDROIDS. 49 



HYDROIDS. 



Under this order, the gciioral character of which has ah-eady 

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

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

 nities in their earlier phas(!s of existence, oi' as free swinuning 

 Medusa) in their farther develoi)ment, ciiallenge our admiration, 

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

 Some of them are so minute that tiicy escaj)e 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 tlie Jelly-fishes upon them. 



To the Hydroids belong the Campannlarians, the Sertularians, 

 and the Tubularians. Some examj)les of each, as re])resented on 

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

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

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

 among Acalephs, under the name of Sii)lionophora3, but included 

 with them here in accordance with the views of Vogt, Agassi/, 

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

 droid communities only in being free and floating, instead of 

 fixed to the ground. Some new fiicts, 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. 



Cam/panularians. 



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

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

 those little shrul)-like comnniiiities of animals called Hydroids, 



* See Chapter on Nanoinia. 

 7 



