312 DECEMBER 



charging siphons of all of that system, into a central 

 main orifice, which rises out of the level with a circular 

 rim, and forms the dark centre of the system. 



In the open ocean there are forms of Tunicata which 

 are not attached, but swim freely; and, what is sur- 

 prising, even compound forms are thus found, progress- 

 ing by a combined action. One of the most curious is 

 the genus Pyrosoma, which consists of long-bodied 

 Ascidians, so united in rings as to constitute a long, 

 free cylindrical tube, closed at one end and open at the 

 other. By the rhythmical contractions and dilatations 

 of the multitude, this great cylinder slowly swims 

 through the open sea. But the most interesting cir- 

 cumstance in its history is that it is intensely luminous, 

 lighting up the midnight ocean with flashes of vivid 

 light, or seen gliding through the dark water like 

 glowing sticks of fire. So it is described by some 

 voyagers, perhaps with a little exaggeration, for Mr. 

 Bennett, to whom we are indebted for many valuable 

 remarks on oceanic zoology, speaks in more subdued 

 tones of it. His account of this and other pelagic 

 phosphorescence is very interesting : 



" On the 8th of June," he observes, " being then in 

 lat. 30 S., and long. 27 5' W., having fine weather and 

 a fresh south-easterly trade- wind, and the range of the 

 thermometer being from 78 to 84, late at night, the 





