PHYSOaKADA. 125 



through the element around it so copiously, that it is difficult to con- 

 ceive whence materials can be derived from which it can be elabo- 

 rated. Of the origin of this fluid we are ignorant, although certain 

 glandular-looking granules, contained in the folds of the pedicle, have 

 been looked upon as connected with its production. 



(325.) We are equally at a loss to account for the production of the 

 irritating secretion, in which the power of stinging seems to reside ; but 

 it is observed that the tentacula seem to be more specially imbued with 

 it than other parts of the body. Perhaps the most remarkable property 

 of the Acalephae is their phosphorescence, to which the luminosity of 

 the ocean an appearance especially beautiful in warm climates is 

 principally due. We have more than once witnessed this phenomenon 

 in the Mediterranean ; and the contemplation of it is well calculated to 

 impress the mind with a consciousness of the profusion of living beings 

 existing around us. The light is not constant, but only emitted when 

 agitation of any kind disturbs the microscopic Medusae which crowd the 

 surface of the ocean : a passing breeze, as it sweeps over the tranquil 

 bosom of the sea, will call from the waves a flash of brilliancy which 

 may be traced for miles ; the wake of a ship is marked by a long track 

 of splendour ; the oars of your boat are raised dripping with living dia- 

 monds ; and if a little of the water be taken up in the palm of the 

 hand and slightly agitated, luminous points are perceptibly diffused 

 through it, which emanate from innumerable little Acalepha3, scarcely 

 perceptible without the assistance of a microscope. All, however, are 

 not equally minute : the Beroes, in which the cilia would seem to be 

 most vividly phosphorescent, are of considerable size ; the Cestum Ve- 

 neris, as it glides rapidly along, has the appearance of an undulating 

 riband of flame several feet in length ; and many of the larger Pulmoni- 

 grade forms shine with such dazzling brightness, that they have been 

 described by navigators as resembling " white-hot shot," visible at some 

 depth beneath the surface. This luminousness is undoubtedly dependent 

 upon some phosphorescent secretion ; but its nature and origin are quite 

 unknown. 



(326.) PHYSOGKADA. In the third division of Acalephse, denominated 

 by Cuvier " Acalephes Hydrostatiques," the body is supported in the 

 water by a very peculiar organ, or set of organs, provided for the pur- 

 pose. This consists of one or more bladders, capable of being filled 

 with air at the will of the animal, which are appended to the body in 

 various positions, so as to form floats of sufficient buoyancy to sustain 

 the creature upon the surface of the sea when in a state of distension, 

 but, when partially empty, allowing it to sink and thus escape the ap- 

 proach of danger. In Physalia (fig. 61), known to sailors by the name 

 of the " Portuguese man-of-war," the swimming-bladder is single, and 

 of great proportionate size, so that when full of air it is exceedingly 

 buoyant, and floats conspicuously upon the waves. The top of this 



