46 HYDROIDS AND JELLY-FISH 



by platforms or tabulae. They are reef-builders, and contribute to 

 the solidity of the coral reef structure. Free-swimming minute 

 sexual medusae are produced, which swim away from the parent 

 colony, and having deposited their eggs, shrivel up and die. The 

 second family, the Stylasters, are remarkable for the elegance and 

 beauty of their branching fan-shaped forms, and exquisite colour- 

 ing. Their branches are covered with numerous groups of pores, 

 each pore having a calcareous spiny projection or style. Numer- 

 ous blister-like swellings or ampullae on the surface of the colony 

 contain the generative buds, which, however, never become free 

 medusae. 



The Siphonophora l are free-swimming Hydrozoa, each con- 

 sisting of a colony or assemblage of individuals united in a com- 

 mon stock (called a hydrosoma), and placed under a more or less 

 tough, bladder-like bag, which acts as a float. To this order 

 belongs the well-known " Portuguese Man-of-war " (Physalia), 

 whose air-sac is in the form of a somewhat pear-shaped bladder, 

 pointed at one end and rounded at the other, crowned by a long, 

 low crest which acts as a sail. Very often in the tropics a regular 

 fleet of these Portuguese Men-of-war are to be seen sailing along 

 upon the surface of the sea, their coloured floats all well above 

 the water, so that the crest acts as a miniature sail ; while from 

 beneath the float hang an assemblage of polyps, medusa-buds, and 

 long trailing tentacles, sometimes many feet in length, and armed 

 with batteries of stinging cells sufficiently powerful to produce 

 very painful results ; indeed, the natives inhabiting the island of 

 Funafuti are said to have a very great fear of being stung by 

 them. Drifting along on the surface of the sea, these Physalia 

 present a very pretty appearance, and always excite admiration 

 and interest. Sometimes during the summer months they are to 

 be seen off the south-west and west coasts of England. 



Another very beautiful form is called the " By-the-wind Sailor " 

 (Velella), which looks like a dainty circular raft, with an obliquely 

 placed semicircular-shaped sail ; and is just the graceful little craft 

 one might imagine the sea-fairies of old voyaged in. All round 

 the edge of the dainty disk-shaped raft depend a fringe of gaily 

 coloured tentacles, which may be purple, bright blue, or brown in 

 colour. 



. Tube, or siphon-bearers. 



