100 THE VICTORIAN NATURALIST. 



having been observed and described by Pliny. This is Pholas, 

 the rock-boring shell-fish, so called because it burrows into rocks. 

 It has three different sets of organs which give out the light. 



These are only a few of the many luminous forms of lower 

 groups. Many of the smaller Crustacea, such as Copepods, are 

 brilliantly phosphorescent, and some of the Echinoderms. There 

 is even a pelagic species of rotifer which is luminous, this being 

 interesting as one of the earliest luminous animals observed with 

 the'microscope. 



Coming now to a higher group, we find an exceedingly beautiful 

 and interesting form which is often seen floating in mid-ocean — 

 Pyrosoma. This is a free-swimming colony of Tunicates, and is 

 related to the sea-squirts found growing on rocks at the sea-shore. 

 They are usually a few inches to a foot long, but are sometimes 

 much larger. A giant colony was obtained on the Challenger 

 voyage, and Professor Moseley (27) says of it : — "The most beautiful 

 kind of phosphorescence is, however, that of the Ascidian colony 

 Pyrosoma. This, when stimulated by a touch, a shake, or a 

 swirl of the water, gives out a bright globe of bluish light which 

 lasts for several seconds .... and ihen goes out suddenly. 

 A giant Pyrosoma was caught by us in the deep sea trawl. It was 

 like a great sac, with its walls of jelly about an inch in thickness. 

 It was four feet long and ten inches in diameter. When a Pyrosoma 

 is stimulated by having its surface touched the phosphorescent 

 light breaks out just at the spot stimulated, and then spreads 

 over the surface of the colony to the surrounding animals. I 

 wrote my name with my finger on the surface of the giant 

 Pyrosoma as it lay on deck, and my name came out in a few 

 seconds in letters of fire." 



The colony is composed of hundreds of animals imbedded in a 

 common jelly-like mass. The light is given out from two distinct 

 patches of cells in each individual, and Panceri (28), who first 

 observed the right use of these cells, states that they are connected 

 by muscular bands, which pass the stimulus on from one 

 individual to another. 



Comparatively little was known about animals living in the 

 depths of the sea until ships specially fitted for dredging and for 

 surveying the seas generally were sent out from Great Britain 

 about the middle of last century. Of these the Challenger was 

 the best fitted for the work, and had the most extended voyages, 

 and our scientific knowledge of deep-sea life really dates from the 

 sailing of that vessel from England in 1872. 



In considering these forms we have to remember that the con- 

 ditions in deep sea are very different from those of shallower 

 water. No sunlight can penetrate below 500 fathoms, to take an 

 extreme limit, and there is also an enormous pressure and a very 

 low temperature. These strange conditions aff"ect the animals 



