86 OCEANIC HYDROZOA. PART m. 



cavity in the main stem. Each individual polype, some- 

 times to the number of nineteen, after having digested 

 its food or prey, ejects the refuse from its mouth, and 

 the nutritious juice traverses the labyrinth of tubes to 

 that general reservoir. 



Since every portion of the bodies of the Hydrse is 

 nearly of the same kind, and as every part of their sur- 

 face inside and outside is in contact with the water in 

 which they live, and from whence they derive oxygen to 

 aerate their juices, no circulation is necessary in these 

 simple animals, either for nutrition of their tissues, or to 

 furnish them with oxygen. 



If the Hydrse only produced deciduous buds which 

 are developed into facsimiles of their parent, their race 

 would become extinct, since they die in winter, unless 

 kept artificially in water of mild temperature ; but the 

 animals are hermaphrodite, so that each individual pro- 

 duces fertilized eggs in autumn, which are hatched in 

 spring, so that the Hydra is alternately propagated by 

 deciduous buds and by eggs. The fresh-water hydrse 

 are the only hydroids that are locomotive, all the others 

 being fixed to some solid substance. 



The oceanic Hydrozoa comprehend the three families 

 of Corynidse, Tubulariidse, and Sertulariidse. They are 

 chiefly compound animals, numerous in genera and 

 species, and have great variety of form. They may be 

 simple and slender, they may be creeping or like a bush 

 or tree, more or less compound and regularly branched 

 according to the form of the polypary or tubular sub- 

 stance which unites their numerous hydra-form polypes 

 into one animal. In general they are exceedingly 

 small ; three or four inches in height is quite gigantic. 

 There is scarcely a still clear pool left by the retiring 

 tide among the rocks along the British coasts, that does 

 not abound with these beautiful creatures attached to 

 stones, old shells, or sea-weeds. But they must be 



