340 



colonial, the compound polype stock being attached to rocks, 

 algie, shells, timbers, or other objects of support, by means 

 of a thread like branching rootstock 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 few forms, like Hydractinia polyclina and 

 some Podocoryne, lead a pseudo-vagrant benthonic life, 

 being attached to the shells of gastropods carried about by 

 hermit crabs. Some species, like Bougainrillia friiticosa, 

 prefer a pseudo-planktonic habit, beconnng attached to 

 floating timbers, a similar habit being assumed by the 

 hydroids which live attached to the floating Sargassum. A 

 pseudo-nektonic manner of life may perhaps be considered 

 the habit of Hydrichthys, which lives parasitic upon a fish. 

 Corrmorplm penclula, 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 perhaps never very great. 



A large number of Hydrozoa have a distinct medusiform 

 person, which, when perfect, is pei'haps the best tyjieof aholo- 

 planktouic organism. In a few Hydrozoa — Hydra, Sertu- 

 laria (?)— the medusiform 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 Xarco- 

 and Tracho-Medusae, as well as in some others, only the 

 medusa occurs, the hydroid being suppressed. Compound 

 medusa? occur as M-ell 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. Lucer-naria is an example of an 

 attached medusa. The medusae, whether free or attached, 

 produce the sexual products which give rise to new hydroid 

 colonies or directly to new medusae. The egg develops into 

 a ciliated planula which leads a mero-planktonic existence 

 before it settles down to become a benthonic hydroid, or 

 before it develops into the medusa, A number of hydroids 

 grow attached to rocks and seaweeds, or to bridge piles, in 



