38 GUIDE TO THE CORAL GALLERY. 



HYDROZOA. 



[(*) An asterisk against names of species denotes that specimens in fluid 

 are exhibited in the upright part of Table Case 3.] 



Introduction. 



The exhibited collection of Hydrozoa occupies Cases 2-i at the 

 eastern end of the Coral Gallery. 



The members of the class occur as fixed plant-like forms often of 

 horny texture {Serhdai'ia, &c., Case 3), or as massive or branched 

 coral-like growths {Sfylaster and MiiUjJora, Cases 2, 4), or as 

 transparent free-swimming bell- or disk-shaped organisms, which 

 may l3e simple (Medusa' or Jelly-Fisl), Case 3), or may form colonies 

 composed of variously modified individuals {Sijjhonophora, Case 3). 

 The vast majority of species are marine, but a few live in fresh 

 water. 



The specimens in Case 3 a, b, closely resemble dried seaweeds, but 

 can generally be distinguished from plants by observing, especially 

 with a lens, that a serrated appearance of the branches is due to 

 little horny cups or receptacles ; see, for instance, Diphasia tamarisca* 

 which has unusually large cylindrical cups. In life, each cup 

 contains a polyp with a crown of tentacles surrounding a mouth 

 opening into a stomach cavity. 



In spite of great differences in form amongst the Hydrozoa, a 

 comparatively simple plan of organisation can be traced in all. 



To briefly explain this, an account is given of Hijdra or the 

 Fresh- Water Polyp, a tiny Hydrozoon which lives in ponds attached 

 to water-weeds (Fig. 1a). Hydra, which is green or reddish-brown 

 in colour, according to the species, attains an average height of 

 about one-third of an inch. The little creature alters its shape 

 considerably, being now contracted down to a lump, now expanded 

 into a little column with a circle of thread-like tentacles near the 

 summit. When a tentacle touches some small organism, the latter 

 is paralysed and drawn into the mouth at the top of the column, 

 and thence into the simple stomach-cavity, where it is digested, the 

 remains being evacuated by the mouth. Between the mouth and 



