lOio PRINCIPLES OF STRATIGRAPHY 



these medusae were observed floating in shoals where the water 

 was pure enough to be drinkable. The majority of species have a 

 sedentary benthonic stage, the hydriform stage, which is generally 

 colonial, the compound polyp stock being attached to rocks, algse, 

 shells, timbers, or other objects of support, by means of a thread- 

 like branching root-stock or hydrorhyza, which spreads out over 

 the object of support and from which the individual polyparia arise, 

 each with a distinct stem or hydrocaulus. A fev/ forms, like Hydrac- 

 tinia polyclina and some Podocoryne, are pseudo-vagrant benthos, 

 being attached to the shells of gastropods carried about by hermit 

 crabs. Some species, like Bougainvillia fruticosa, prefer an epi- 

 planktonic habit, becoming attached to floating timbers, a similar 

 habit being assumed by those hydroids which live attached to the 

 floating Sargassum. An epi-nektonic manner of life may perhaps 

 be considered the habit of Hydrichthys, which lives parasitically 

 upon a fish. CoryiiiorpJia pendula, though not attached, lives partly 

 buried in the mud of the shallow sea ; while Hydra leads, at times 

 at least, a kind of vagrant benthonic life, though its journeyings 

 are probably never very great. 



Many, if not most Hydrozoa have a distinct medusiform person 

 which, when perfect, is perhaps the best type of a holo-planktonic 

 organism. In a few Hydrozoa — Hydra Sertularidae — the medusi- 

 form stage is wanting, in others it is degenerate, never becoming 

 free (Clava) ; but in a large number of species it is a free individual. 

 Again, in the Narco- and Trachymedus?e, as well as in some others, 

 only the medusa occurs, the hydroid being suppressed. Compound 

 medusae occur as well as compound hydroids. The former are 

 the Siphonophora in which, by budding from the parent medusa, a 

 compound colony is formed which leads a holo-planktonic existence. 

 Lucernaria is an example of a medusa attached to foreign objects. 

 The medusae, whether free or attached, produce the sexual products 

 which give rise to new hydroid colonies or directly to new medusae. 

 The tgg develops into a ciliated planula which leads a mero-plank- 

 tonic existence before it settles down to become a benthonic 

 hydroid, or before it develops into the medusa. A number of hy- 

 droids grow attached to rocks and sea-weeds, or to bridge piles, in 

 such a position as to become regularly exposed for several hours 

 each day during ebb tide. Even the delicate and unprotected Clava 

 of our northern shores delights to live under such conditions, and 

 is rarely found in deeper water or in tide pools. Most hydroids, 

 however, can not withstand such exposure, and hence they are 

 found only in the deeper waters or the deeper tide pools. 



The majority of hydroids are inhabitants of the littoral district, 



