304 COELENTERATA HYDROZOA chap. 



organisms. Some of them (Auronectidae) appear to have become 

 adapted to a deep-sea habit, others are usually found in 

 intermediate waters, but the majority occur with the pelagic 

 plankton at or very near the surface of the open sea. Although 

 the order may be said to be cosmopolitan in its distribution, the 

 Siphonophora are only found in great numbers and variety in 

 the sub-tropical and tropical zones. In the temperate and arctic 

 zones they are relatively rare, but Galeolaria biloba and Physo- 

 phora horealis appear to be true northern forms. The only 

 British species are Muggiaea atlantica and Cupulita sarsii. 

 Velella spirans occasionally drifts from the Atlantic on to our 

 western shores, and sometimes great numbers of the pneumato- 

 phores of this species may be found cast up on the beach. Diphyes 

 sp., Physalia sp., and Physophora borealis are also occasionally 

 brought to the British shores by the Gulf Stream. 



The Calycophorae are usually perfectly colourless and trans- 

 parent, with the exception of the oil-globule in the oleocyst, 

 which is yellow or orange in colour. Many of the other Siphono- 

 phora, however, are of a transparent, deep indigo blue colour, 

 similar to that of many other components of the plankton. 



Most of the Siphonophora, although, strictly speaking, surface 

 animals, are habitually submerged ; the large pneumatophores of 

 Velella and Physalia, however, project above the surface, and 

 these animals are therefore frequently drifted by the prevailing 

 wind into large shoals, or blown ashore. At Mentone, on the 

 Mediterranean, Velella is sometimes drifted into the harbour in 

 countless numbers. Agassiz mentions the lines of deep blue 

 Velellas drifted ashore on the .coast of Florida ; and a small 

 species of blue Physalia may often be seen in long lines on the 

 shore of some of the islands of the Malay Archipelago. 



The food of most of the Siphonophora consists of small 

 Crustacea and other minute organisms, but some of the larger 

 forms are capable of catching and devouring fish. It is stated by 

 Bigelow ^ that a big Physalia will capture and devour a full- 

 grown Mackerel. The manner in which it feeds is described as 

 follows : — -" It floats on the sea, quietly waiting for some helpless 

 individual to bump its head against one of the tentacles. The fish, 

 on striking, is stung by the nettle-cells, and fastened probably by 

 them to the tentacle. Trying to run away the fish pulls on the 



^ Johns Hopkins Univ. Giro. x. 1891, p. 91. 



