ON A FRKE-SWIMiMING HYDBOID. 3 



2. Notes on thr Living Animal. 



The free-swimming hydroid person of Pel ago hydra 

 mirabilis (fig. 1) is apparently a pelagic organism. The 

 conditions under which it was found, its subsequent be- 

 haviour when observed in sea water, and its pecnliar organi- 

 sation, all point to this conclusion. When placed in a glass 

 of sea water in front of a candle (it was too dark to examine 

 it by daylight) it floated near the surface with the narrow 

 proboscis-like portion of the body, beai'ing the mouth at its 

 extremity, hanging downwards from the much larger balloon- 

 like structure, which I propose to call the 'Afloat." The 

 latter, though near the surface, was totally submerged. 

 Subsequently, when placed in a tin can for removal to the 

 laboratory and kept in the dark, the animal sank to the 

 bottom, though still alive. Probably, therefore, it has the 

 power of rising and sinking in the water like other pelagic 

 organisms, and it may be that it always sinks to some depth 

 beneath the surface when it is dark. The general colour of 

 the organism was a very pale bluish tint, and it was of 

 course translucent. The proboscis, however, was pale pink, 

 intensified round the margin of the mouth. The manubria 

 of the medusoids were also pink. During life the hydroid 

 exhibited some slight power of changing its shape, the float 

 being at one time oval (slightly elongated vertically) and 

 at another contracted into a sphere, while the proboscis 

 exhibited considerable power both of contraction, under 

 which condition it became slightly trumpet-shaped at the 

 end, and of flexion. When both elongated, as shown in 

 fig. 1, the float was nearly an inch in greater diameter and 

 the proboscis rather more than half as long as the float. 



The long, slender, tentacular processes of the float occa- 

 sionally exhibited spasmodic movements of flexion, like 

 gigantic flagella, many of them simultaneously, or neai'ly so; 

 and from this I am led to conclude that the animal has the 

 power of rowing itself through the water by means of these 

 organs. 



