350 EAMBLES OF A NATURALIST. [Ch. XXI. 



observed upon the surface ? Why are they not more fre- 

 quently seen ? and why are the occasions so rare in which 

 they are observed in such profusion ? There must be cir- 

 cumstances connected either with their physical constitution 

 or their modes of obtaining food, with which we are entirely 

 unacquainted, but which must materially influence their 

 movements. Doubtless they are sunk below the surface a 

 short distance when not seen,- for we cannot suppose that, 

 short-lived as they may often be, they are suddenly produced 

 like a crop of mushrooms in damp weather. They must exist 

 somewhere, and a common influence probably regulates, their 

 movements, which perhaps need be but slight to bring them 

 into view, or to carry them once more out of sight. 



I never obtained the Pyrosoma in the towing-net, nor 

 did I ever see them floating upon the surface. Yet these 

 oceanic animals doubtless abound, and if I am right in 

 attributing certain luminous appearances to them, they 

 must most commonly float at a distance of two or three 

 fathoms below the surface, though I have on one or two 

 occasions seen the luminous body whirled to the surface in 

 the eddy of the ship's wake. 



With regard to the Hydrozoa of the order Lucemaridse (the 

 covered-eyed Medusae of Forbes), on the comparatively few 

 occasions when they appeared upon the surface, they were 

 usually in great abundance, and not in great variety. Thus 

 in the upper part of the Eed Sea, on the 10th of March, a 

 species of Aurelia appeared in great numbers ; and two days 

 after, we passed through a shoal of Ehizostomas. Four 

 days later, in the Gulf of Aden, we again encountered shoals 

 of Aurelia, apparently identical with those of the Eed Sea, 

 the two shoals being separated by about 1400 mUes. Again, 

 in October, we passed, on the west coast of Borneo, off 



