130 THE SEAS 



they should be able to maintain the level at which they are 

 drifting. The means by which this capacity is attained are 

 many and varied, the general aim being to reduce the 

 organism's specific gravity directly until it is the same as or 

 less than, that of the surrounding sea water, or to obtain a 

 similar effect in a more indirect manner. 



Species that have achieved the power of becoming lighter 

 than water are comparatively few in number. The chief 

 examples occur in a group of jellyfishes known as Siphono- 

 phores. The name of the Portuguese Man-o'-War, a 

 stinging jellyfish, is well-known to all. This animal 

 possesses a specially designed " float " into which gas is 

 secreted by a specialized gland. This gas-filled reservoir 

 projects above the surface of the sea and acts as a sail, by 

 means of which the wind blows the jellyfish along, trans- 

 porting it from place to place with tentacles extended in all 

 directions ready to seize any unwary prey that they may 

 touch, instantly paralysing it with their batteries of stinging 

 cells. Another closely allied form is the Velella, or " By 

 the wind sailor " (Plate 50). This likewise has a small 

 gas-filled sail. The animal, when seen alive, is of a fairy- 

 like delicacy, possessing this transparent, papery sail, 

 situated above the centre of the body : on the under 

 surface is the " mouth," centrally placed and surrounded by 

 delicate mobile tentacles of a sky-blue tint. These little 

 creatures, which reach a size of one or two inches in length, 

 are natives of the warmer ocean waters and the Mediter- 

 ranean. However, they are occasionally to be found 

 stranded along the western and south-western shores of 

 the British Isles after prolonged southerly winds, that have 

 wafted them speedily along the surface of the sea from the 

 warmer latitudes. 



In order to reduce the specific gravity as nearly as possible 

 to that of sea water, many animals make use of fats and 



