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THE POPULAR SCIENCE MONTHLY. 



to another, and arrested at all stages of development. Sometimes, 

 although completely formed, they resign their freedom and end their 

 existence in the colony where they were bom. 



In one group of Polyps the Medusse associate themselves with the 

 reproductive individuals to form a new unit — a small, distinct colony, 

 that might be taken for a peculiar organ curiously analogous to a 



Fig. 4.— Gbnerative Buds or Gonophoees of the Eydrozoa diagrammatically represented. 

 a, simple gonophore; c, gonophoie which has the structure of a Medusa (medusoid), but is not 

 detached ; d, free medusiform gonophore. 



flower — with a separate chamber, and called the gonangium (Figs. 3 

 and 4). A step further and these strongly individualized Medusae are 

 seen descending to the rank of organs in more complex colonies. 



All the colonies of Hydras are not fixed to submarine objects. 

 Some of them lead a vagabond existence. They are often taken, not 

 without reason, for simple animals analogous to the Medusae, and called 



Fig. 5.— oceanic Hydrozoa, showing the specialization of Parts. 9, Siphonophore ; w, 

 swimming-bells ; p, alimentary region ; t, tentacles ; 3, diagram of the composite body of one 

 of the Siphonifera i a a, swimming-bells ; d, spines or defensive individuals ; /, digester. 



Siphonophores (Fig. 5). They sometimes attain a large size ; and the 

 variety and profusion of the parts which compose them, as well as the 

 brilliancy of color and incomparable beauty of their forms, have made 

 them subjects of the profound admiration of naturalists as well as 

 sailors (Figs. 6 and 7). Each one of these parts is the equivalent of a 



