PELAGIC FAUNA 



75 



the warmer seas. One of these is the Por- 

 tuguese man-of-war {Physalia, Fig. adjoin- 

 ing), with a large parchment-hke bladder, 

 magnificently coloured. The various indivi- 

 duals are suspended in the water from the 

 underside of the bladder. The tentacle 

 members of the colony may be many metres 

 in length and will impart a virulent sting. 

 In the by-the-wind-sailor [Velella, Fig. p. 

 74) the place of the bladder is taken by a 

 multi-chambered, oval-shaped disk. On the 

 summit is an oblique crest, and when this 

 "sail" is caught by the wind the whole co- 

 lony is propelled like a ship sailing by the 

 wind. It is sometimes attacked by one of 

 the isopod crustaceans, the richly coloured 

 little Idothea metallica. In an aquarium off 

 South Africa we observed four of these 

 small crustaceans devour a by-the-wind- 

 sailor three centimetres long in 12 hours. 

 Another sophonophore, Porpita, lacks the 

 sail but has a circular disk. Usually, this 

 creature is a beautiful blue; but in the Java 

 Sea, besides the blue ones, we saw some 

 that were a bright lemon colour. 



The jellyfishes which were studied most 

 of all during my stay on board were the 

 medusae. Most of these were readily identi- 

 fied, and it was useful to see them alive 

 and in their natural forms and beautiful 

 colours, which are mostly lost by preserva- 

 tion. Before reaching Australia we had 

 found nearly 150 different species of me- 

 dusae, 20 of them previously unknown. We 

 were able to clear up many doubtful points 

 about the demarcation of species, and also 

 in a number of cases to describe the various 

 stages of development. Many of these 



The Portuguese man-of-war (Physalia). 



^' 



