PHOSPHORESCENCE 33 



expedition also secured adult examples of the wonder- 

 ful free-swimming holothurian, Pelagothuria ludwigi, 

 which so curiously mimics a jelly-fish. It was taken 

 in a closing-net at 400 to 500 fathoms near the Sey- 

 chelles. Most of these animals bear their origin 

 stamped on their structure, so that a zoologist can 

 readily pick out from a miscellaneous collection of 

 forms those which have a deep-sea home. We have 

 already referred to a certain ' stalkiness,' which lifts 

 the fixed animals above the slowly deepening ooze. 

 Possibly the long-knobbed tentacles of the deep-sea 

 jelly-fish, Pedis, on the tips of which it is thought the 

 creature moves about, may be connected with the 

 same cause. The great calm of the depths and its 

 effect upon the symmetry of the body have also been 

 mentioned ; but greater in its effect on the bodies 

 of the dwellers in the ocean abysses is the absence 

 of sunlight. 



No external rays reach the bottom of the sea, and 

 what light there is must be supplied by the phos- 

 phorescent organs of the animals themselves, and 

 must be faint and intermittent. A large percentage 

 of animals taken from the deep sea show phos- 

 phorescence when brought on deck; and it may be 

 that this emission of light is much greater at a low 

 temperature, and under a pressure of i to 2 tons 

 on the square inch, than it is under the ordinary 

 atmospheric conditions of the surface. The simplest 

 form which these phosphorescent organs take is that 

 of certain skin-glands which secrete a luminous slime. 

 Such a slime is cast off, according to Filhol, by many 

 of the annelids ; and a similar light-giving fluid is 

 exuded from certain glands at the base of the antenna 

 and elsewhere in some of the deep-sea shrimps. But 

 the most highly developed of the organs which pro- 

 duce light are the curious eye-like lanterns which 

 form one or more rows along the bodies of certain 



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